dts: bindings: scmi: Update shmem from phandle into phandles

Extends the SCMI shared memory structure to phandles,
allowing TX and RX channels to use different shared memory regions.
This is necessary for platforms like i.MX95, where TX and RX
notifications are mapped to different addresses.

RX channel is not mandatory and depends on the platform's support

Signed-off-by: Yongxu Wang <[email protected]>

Author: yongxu-wang15

dts/bindings/firmware/arm,scmi.yaml

@@ -48,11 +48,12 @@ include: [base.yaml]
 
 properties:
   shmem:
-    type: phandle
+    type: phandles
     required: true
     description: |
-      Phandle to node describing TX channel shared memory area.
-      This translates to a **single** SCMI transmit channel.
+      Phandle array to nodes describing shared memory areas for SCMI transport.
+      The first entry corresponds to the TX channel, and the second (if present)
+      corresponds to the RX channel.
 
   mboxes:
     required: true
@@ -62,6 +63,8 @@ properties:
         mailbox channel for signaling)
         2) tx_reply - 2 mbox / 1 shmem (platform and agent use different
         mailbox channels for signaling)
+        3) tx + rx - 2 mbox / 2 shmem (platform and agent use different mailbox
+        channels and shmem between scmi_command and scmi_notification)
 
   mbox-names:
     required: true