drivers/mtd/nvmxip: introduce NVM XIP block storage emulation

add block storage emulation for NVM XIP flash devices

Some paltforms such as Corstone-1000 need to see NVM XIP raw flash
as a block storage device with read only capability.

Here NVM flash devices are devices with addressable
memory (e.g: QSPI NOR flash).

The implementation is generic and can be used by different platforms.

Two drivers are provided as follows.

  nvmxip-blk :

    a generic block driver allowing to read from the XIP flash

  nvmxip Uclass driver :

        When a device is described in the DT and associated with
        UCLASS_NVMXIP, the Uclass creates a block device and binds it with
	 the nvmxip-blk.

Platforms can use multiple NVM XIP devices at the same time by defining a
DT node for each one of them.

Signed-off-by: Abdellatif El Khlifi <abdellatif.elkhlifi@arm.com>

include/dm/uclass-id.h

@@ -89,6 +89,7 @@ enum uclass_id {
 	UCLASS_NOP,		/* No-op devices */
 	UCLASS_NORTHBRIDGE,	/* Intel Northbridge / SDRAM controller */
 	UCLASS_NVME,		/* NVM Express device */
+	UCLASS_NVMXIP,		/* NVM XIP devices */
 	UCLASS_P2SB,		/* (x86) Primary-to-Sideband Bus */
 	UCLASS_PANEL,		/* Display panel, such as an LCD */
 	UCLASS_PANEL_BACKLIGHT,	/* Backlight controller for panel */