dt-bindings: firmware: arm,scpi: Add missing maxItems to mboxes/shmem

"make dt_binding_check":

Documentation/devicetree/bindings/firmware/arm,scpi.example.dt.yaml: scpi: shmem: [[2], [1]] is too long

Fix this by adding a proper maxItems value to the shmem property,
and to the related mboxes property.

Fix the grouping of the "mboxes" property in the example.

Signed-off-by: Geert Uytterhoeven <[e-mail]>
Acked-by: Sudeep Holla <[e-mail]>
Signed-off-by: Rob Herring <[e-mail]>
Link: https://lore.kernel.org/r/58b7c[e-mail]

Documentation/devicetree/bindings/firmware/arm,scpi.yaml

···
       by remote SCP firmware for use by SCPI message protocol should be
       specified in any order.
     minItems: 1
+    maxItems: 4
 
   shmem:
     description:
···
       be any memory reserved for the purpose of this communication between the
       processors.
     minItems: 1
+    maxItems: 4
 
   power-controller:
     type: object
···
     firmware {
         scpi {
             compatible = "amlogic,meson-gxbb-scpi", "arm,scpi-pre-1.0";
-            mboxes = <&mailbox 1 &mailbox 2>;
+            mboxes = <&mailbox 1>, <&mailbox 2>;
             shmem = <&cpu_scp_lpri>, <&cpu_scp_hpri>;
 
             scpi_sensors1: sensors {