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-S2HPSigned-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/watchdog/brcm,bcm2835-pm-wdog.txt

-BCM2835 Watchdog timer
-
-Required properties:
-
-- compatible : should be "brcm,bcm2835-pm-wdt"
-- reg : Specifies base physical address and size of the registers.
-
-Optional properties:
-
-- timeout-sec   : Contains the watchdog timeout in seconds
-
-Example:
-
-watchdog {
-	compatible = "brcm,bcm2835-pm-wdt";
-	reg = <0x7e100000 0x28>;
-	timeout-sec = <10>;
-};