ArmPlatformPkg: Add PCD for SBSA Watchdog Count

ArmPlatformPkg: Add PCD for SBSA Watchdog Count

The Juno and FVP platform implement the SBSA Watchdog timers.
Added PcdWatchdogCount to specify the number of Watchdog timers
that are available.

This allows configurability and an option to disable the watchdog
timers if required for testing.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Sami Mujawar <sami.mujawar@arm.com>
Signed-off-by: Evan Lloyd <evan.lloyd@arm.com>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>