Silicon/Qemu/Sbsa: Add SBSA-wdt entry to GTDT

SBSA generic watchdog timer structure entry has been added
to GTDT table as per SBSAv6.0.
This enables acpi detection of wdt in qemu sbsa platform

Cc: Leif Lindholm <leif@nuviainc.com>
Cc: Ard Biesheuvel <ard.biesheuvel@arm.com>
Cc: Graeme Gregory <graeme@nuviainc.com>
Signed-off-by: Shashi Mallela <shashi.mallela@linaro.org>
Reviewed-by: Graeme Gregory <graeme@nuviainc.com>
Acked-by: Leif Lindholm <leif@nuviainc.com>