Platform/Sgi: add SMMU and timer entries to memory description table

Add PCDs for base address and address space size for generic timer and
SMMU controllers. Use those PCDs to add platform memory map entries. The
ServerReady SBSA tests, when executed, accesses these controllers and so
the memory mapping for generic timer and SMMU controllers are required.

In addition to this, PCDs for watchdog timer controller base address and
size are introduced instead of using macros for the same. This allows
the base address and address space size for watchdog timer controller to
be specified by platform description files.

Signed-off-by: Pranav Madhu <pranav.madhu@arm.com>
Reviewed-by: Sami Mujawar <sami.mujawar@arm.com>