dt-bindings: soc: microchip: add a property for system controller flash

The system controller "shares" a SPI flash device with a QSPI controller
in the MSS. This flash is used to store FPGA bitstreams & other
metadata. IAP and Auto Upgrade both write images to this flash that the
System Controller will use to re-program the FPGA.

Add a phandle property signifying which flash device is connected to the
system controller.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>

Author: ConchuOD

Documentation/devicetree/bindings/soc/microchip/microchip,mpfs-sys-controller.yaml

@@ -26,6 +26,16 @@ properties:
   compatible:
     const: microchip,mpfs-sys-controller
 
+  microchip,bitstream-flash:
+    $ref: /schemas/types.yaml#/definitions/phandle
+    description:
+      The SPI flash connected to the system controller's QSPI controller.
+      The system controller may retrieve FPGA bitstreams from this flash to
+      perform In-Application Programming (IAP) or during device initialisation
+      for Auto Update. The MSS and system controller have separate QSPI
+      controllers and this flash is connected to both. Software running in the
+      MSS can write bitstreams to the flash.
+
 required:
   - compatible
   - mboxes