dt-bindings: mfd: Add Broadcom's Timer-Watchdog block

It's a block implementing few time related functions depending on a
(SoC specific) variant. At this point there is ready binding for a
watchdog only. Work on remaining subblocks (e.g. "reg" based reboot) is
in progress.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20211115055354.6089-2-zajec5@gmail.com