dts: bindings: memc: add a max-frequency property for stm32 xspi psram

Add a parameter to define the maximum PSRAM frequency for the STM32 XSPI
driver. It will be useful to automatically calculate a prescaler.

Signed-off-by: Guillaume Gautier <guillaume.gautier-ext@st.com>

Author: gautierg-st

dts/bindings/memory-controllers/st,stm32-xspi-psram.yaml

@@ -17,6 +17,11 @@ properties:
     required: true
     description: Flash Memory size in bits
 
+  max-frequency:
+    type: int
+    required: true
+    description: Maximum clock frequency of device's xSPI interface in Hz
+
   fixed-latency:
     type: boolean
     description: |