schemas: reserved-memory: Add export property

OSs might want to create dedicated, special purposes, memory allocators
from memory regions with different memory attributes or characteristics,
for the rest of the system to consume.

Let's add an export property to allow reserved memory regions to be used
as the backing memory for such allocators.

Signed-off-by: Maxime Ripard <mripard@kernel.org>

dtschema/schemas/reserved-memory/reserved-memory.yaml

@@ -66,6 +66,12 @@ properties:
       which can be corrected by the Error-Correction Code (ECC) memory
       subsystem (typically 0, 1 or 2).
 
+  export:
+    type: boolean
+    description: >
+      The operating system can use the memory to create special-purpose
+      allocators instead of solely dedicating it to the device.
+
   iommu-addresses:
     $ref: /schemas/types.yaml#/definitions/phandle-array
     description: >