doc: board: nxp: Add Quickboot documentation

Add instructions on how to use U-Boot to save
DDR training data to NVM and explain the saving
process.

Signed-off-by: Simona Toaca <simona.toaca@nxp.com>

doc/board/nxp/index.rst

@@ -30,3 +30,4 @@ NXP Semiconductors
    mx6ullevk
    rproc
    psb
+   quickboot

doc/board/nxp/quickboot.rst

@@ -0,0 +1,59 @@
+.. SPDX-License-Identifier: GPL-2.0+
+   Copyright 2026 NXP
+
+DDR QuickBoot flow
+------------------
+
+Some NXP SoCs (which use OEI - iMX943, iMX95, iMX952 etc.) support saving
+DDR training data (collected by OEI during Training flow) from volatile
+to non-volatile memory, which is then available to OEI at next cold reboot.
+OEI uses the saved data to run Quickboot flow and avoid training the DDR again.
+This significantly reduces the boot time.
+
+The location of the quickboot data in NVM is a space left in the bootloader by
+mkimage, with the size of 64K. The qb command searches for this space to
+save the data. Thus, the NVM should also be a boot device and contain
+the bootloader at the time of the saving.
+
+U-Boot provides no authentication for quickboot data, only its integrity
+is verified via the CRC32. The authentication is done in OEI. With
+the exception of iMX95 A0/A1, which use CRC32 as well for verifying
+the data, the rest of the SoCs use ELE to verify the MAC stored
+in the ddrphy_qb_state structure.
+
+If the quickboot data in memory is not valid (CRC32 check fails),
+U-Boot does not save it to NVM. So, if OEI runs Quickboot flow -> no
+data is written to volatile memory -> invalid data -> no saving happens
+(qb save fails during qb check).
+
+After successful saving, U-Boot clears the data in volatile memory so
+that qb check fails at next reboot and the NVM isn't accessed again.
+
+There are 2 ways to save this data, both can be enabled:
+
+1. automatically, in SPL (by enabling CONFIG_SPL_IMX_QB)
+
+- this will save the data on the current boot device (e.g. SD)
+- other configs specific to the boot device need to be enabled (CONFIG_SPL_MMC_WRITE for saving to eMMC/SD)
+- use for: automating qb save / saving quickboot data if using Falcon mode (skipping U-Boot proper)
+
+2. using qb command in U-Boot console (by enabling CONFIG_CMD_IMX_QB)
+
+- supports saving on the current boot device, or on another, specified device.
+- supports specifying the hwpartition for eMMC (for booting from boot0/boot1)
+- if flashing via uuu, the command can be added in an uuu script (boot device needs to be specified)
+- use 'qb erase' to force DDR re-training
+- use for: saving quickboot data during flashing / controlling the NVM to save to / forcing re-training
+
+::
+
+        # To save/erase on current boot device
+        # For eMMC boot1, mmc 0:2 has to be specified explicitly
+        => qb save/erase
+
+        # To save/erase on other boot device
+        => qb save/erase mmc 0   # eMMC boot0
+        => qb save/erase mmc 0:1 # eMMC boot0
+        => qb save/erase mmc 0:2 # eMMC boot1
+        => qb save/erase mmc 1   # SD
+        => qb save/erase spi     # NOR SPI