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>
diff --git a/doc/develop/driver-model/nvmxip.rst b/doc/develop/driver-model/nvmxip.rst
new file mode 100644
index 0000000..fe087b1
--- /dev/null
+++ b/doc/develop/driver-model/nvmxip.rst
@@ -0,0 +1,48 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+NVM XIP Block Storage Emulation Driver
+=======================================
+
+Summary
+-------
+
+Non-Volatile Memory devices with addressable memory (e.g: QSPI NOR flash) could
+be used for block storage needs (e.g: parsing a GPT layout in a raw QSPI NOR flash).
+
+The NVMXIP Uclass provides this functionality and can be used for any 64-bit platform.
+
+The NVMXIP Uclass provides the following drivers:
+
+      nvmxip-blk block driver:
+
+        A generic block driver allowing to read from the XIP flash.
+	The driver belongs to UCLASS_BLK.
+	The driver implemented by drivers/mtd/nvmxip/nvmxip.c
+
+      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.
+	The Uclass driver implemented by drivers/mtd/nvmxip/nvmxip-uclass.c
+
+    The implementation is generic and can be used by different platforms.
+
+Supported hardware
+--------------------------------
+
+Any 64-bit plaform.
+
+Configuration
+----------------------
+
+config NVMXIP
+	  This option allows the emulation of a block storage device
+	  on top of a direct access non volatile memory XIP flash devices.
+	  This support provides the read operation.
+	  This option provides the block storage driver nvmxip-blk which
+	  handles the read operation. This driver is HW agnostic and can support
+	  multiple flash devices at the same time.
+
+Contributors
+------------
+   * Abdellatif El Khlifi <abdellatif.elkhlifi@arm.com>