dts: bindings: flash controller size of the stm32 xspi nor

This PR adds the size in Bits of the flash nor memory
for the st,stm32-xspi-nor compatible

Signed-off-by: Francois Ramu <francois.ramu@st.com>

Author: FRASTM

dts/bindings/flash_controller/st,stm32-xspi-nor.yaml

@@ -23,6 +23,9 @@ include:
       - spi-bus-width
       - data-rate
 properties:
+  size:
+    required: true
+    description: Flash Memory size in bits
   spi-bus-width:
     type: int
     required: true