ArmPkg/GenericWatchdogDxe: Split 64bit register write to 2x32bit

ArmPkg/GenericWatchdogDxe: Split 64bit register write to 2x32bit

According to the SBSA specification the Watchdog Compare
Register is split into two separate 32bit registers.
EDK2 code uses a single 64bit transaction to update
them, which can be problematic, depending on the SoC
implementation and could result in unpredictable behavior.

Fix this by modifying WatchdogWriteCompareRegister routine to
use two consecutive 32bit writes to the Watchdog Compare Register
Low and High, using new dedicated macros.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Marcin Wojtas <mw@semihalf.com>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>