ArmPkg/GenericWatchdogDxe: implement RegisterHandler() method

Even though UEFI does not appear to use it, let's implement the
complete PI watchdog protocol, including handler registration,
which will be invoked before the ResetSystem() runtime service
when the watchdog timer expires.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>