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

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>