dt-bindings: mailbox: Convert omap-mailbox.txt binding to YAML

Convert the current OMAP Mailbox binding from text format to YAML
format/DT schema, and delete the legacy text binding file.

The new YAML binding conversion is an updated version compared to
the original. The descriptions for certain properties have been
improved to provide more clarity. Constraints are added to the
properties 'ti,mbox-num-users', 'ti,mbox-num-fifos' and 'interrupts'.
The 'ti,hwmods' is a legacy property and is retained only to reflect
the existing usage on some older OMAP2 and OMAP3 platforms.

All the existing examples have also been updated to reflect the
latest dts nodes (ti,hwmods removed from OMAP4 and AM33xx examples,
and interrupts value updated for AM65x SoCs).

Signed-off-by: Suman Anna <s-anna@ti.com>
[robh: Update ref in ti,omap-remoteproc.yaml]
Link: https://lore.kernel.org/r/20210520234348.4479-1-s-anna@ti.com


Signed-off-by: Rob Herring <robh@kernel.org>