Silicon/Qemu: use dynamic Pcds for SbsaQemu GIC addresses

The GIC addresses as currently declared as FixedPcd for SbsaQemu.
Change them to dynamic, to enable future patches to support these
being determined at runtime.

Signed-off-by: Leif Lindholm <quic_llindhol@quicinc.com>
Reviewed-by: Graeme Gregory <graeme@xora.org.uk>
Reviewed-by: Sami Mujawar <sami.mujawar@arm.com>
Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
Cc: Graeme Gregory <graeme@xora.org.uk>
Cc: Radoslaw Biernacki <rad@semihalf.com>
Cc: Marcin Juszkiewicz <marcin.juszkiewicz@linaro.org>