Silicon/SbsaQemu: generate GTDT from C

We need to have some conditional code while generating GTDT due
to adding non-secure EL2 virtual timer.

Signed-off-by: Marcin Juszkiewicz <marcin.juszkiewicz@linaro.org>
Reviewed-by: Leif Lindholm <quic_llindhol@quicinc.com>