imx8mp_evk: Add a README file

Add a README file explaining the U-Boot build and SD card flash procedures.

Signed-off-by: Fabio Estevam <festevam@gmail.com>

board/freescale/imx8mp_evk/README

@@ -0,0 +1,41 @@
+U-Boot for the NXP i.MX8MP EVK board
+
+Quick Start
+===========
+- Build the ARM Trusted firmware binary
+- Get the firmware-imx package
+- Build U-Boot
+- Boot
+
+Get and Build the ARM Trusted firmware
+======================================
+Note: $(srctree) is the U-Boot source directory
+Get ATF from: https://source.codeaurora.org/external/imx/imx-atf
+branch: imx_5.4.3_2.0.0
+$ make PLAT=imx8mp bl31
+$ sudo cp build/imx8mp/release/bl31.bin $(srctree)
+
+Get the ddr firmware
+====================
+$ wget https://www.nxp.com/lgfiles/NMG/MAD/YOCTO/firmware-imx-8.7.bin
+$ chmod +x firmware-imx-8.7.bin
+$ ./firmware-imx-8.7
+$ sudo cp firmware-imx-8.7/firmware/ddr/synopsys/lpddr4_pmu_train_1d_dmem_201904.bin $(srctree)/lpddr4_pmu_train_1d_dmem.bin
+$ sudo cp firmware-imx-8.7/firmware/ddr/synopsys/lpddr4_pmu_train_1d_imem_201904.bin $(srctree)/lpddr4_pmu_train_1d_imem.bin
+$ sudo cp firmware-imx-8.7/firmware/ddr/synopsys/lpddr4_pmu_train_2d_dmem_201904.bin $(srctree)/lpddr4_pmu_train_2d_dmem.bin
+$ sudo cp firmware-imx-8.7/firmware/ddr/synopsys/lpddr4_pmu_train_2d_imem_201904.bin $(srctree)/lpddr4_pmu_train_2d_imem.bin
+
+Build U-Boot
+============
+$ export CROSS_COMPILE=aarch64-poky-linux-
+$ make imx8mp_evk_defconfig
+$ export ATF_LOAD_ADDR=0x960000
+$ make flash.bin
+
+Burn the flash.bin to the MicroSD card at offset 32KB
+$sudo dd if=flash.bin of=/dev/sd[x] bs=1K seek=32; sync
+
+Boot
+====
+Set Boot switch to SD boot
+Use /dev/ttyUSB2 for U-Boot console