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 <geert+renesas@glider.be>
Acked-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/58b7c706f259f88a61bfe82d9106fe0a93a9838d.1646761693.git.geert+renesas@glider.be