dt-bindings: mailbox: Convert brcm,iproc-flexrm-mbox to DT schema
Convert the Broadcom FlexRM Ring Manager binding to DT schema format.
It's a straightforward conversion.
Link: https://lore.kernel.org/r/20250812181415.66923-1-robh@kernel.org
Signed-off-by: Rob Herring (Arm) <robh@kernel.org>