dt-bindings: rtc: Convert Broadcom STB waketimer to YAML

Convert the Broadcom STB waketimer Device Tree binding to YAML to help
with validation.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/20211208003727.3596577-5-f.fainelli@gmail.com


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