ArmPkg/GenericWatchdogDxe: Set up the watchdog timeout relatively to the system counter

ArmPkg/GenericWatchdogDxe: Set up the watchdog timeout relatively to the system counter

Use the system counter to compute the watchdog compare
value as the watchdog compare value is compared to the
system counter to check if the watchdog timeout expired or not.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ronald Cron <Ronald.Cron@arm.com>
Reviewed-by: Olivier Martin <olivier.martin@arm.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@16513 6f19259b-4bc3-4df7-8a09-765794883524