dt-bindings: watchdog: drop obsolete brcm,bcm2835-pm-wdt bindings

These bindings are already (better) described in soc/bcm/brcm,bcm2835-pm.yaml.
Drop these obsolete bindings.

Signed-off-by: Stanislav Jakubek <stano.jakubek@gmail.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/ZdC/624d1c8O3NRG@standask-GA-A55M-S2HP


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