dt-bindings: soc: Add a new binding for the BCM2835 PM node. (v4)

This binding supersedes the bcm2835-pm-wdt binding which only covered
enough to provide a watchdog, but the HW block is actually mostly
about power domains.

Signed-off-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Rob Herring <robh@kernel.org> (v3)
Acked-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>