fix(morello): dts: fix SCMI shmem/mboxes grouping

The official Arm MHU DT binding suggests to group the shmem (and mboxes)
values to signify the number of mailboxes supported.
Linux' "make dtbs_check" reports:
============
.../morello-soc.dt.yaml: scmi: shmem:0: [17, 18] is too long
        From schema: dt-schema.git/dtschema/schemas/mbox/mbox-consumer.yaml
============

Add angle brackets at the right location to mark the boundaries between
the two mailbox instances used.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Change-Id: If585c98b5e8e55cd5c0b1261e03ce4b91a4c0413
2 files changed