Platform/Sgi: Enable PrimeCell GPIO

The HW-Reduced ACPI model has specific requirements for GPIO
controllers. Arm's reference design Platforms has PrimeCell GPIO
(PL061) integrated in the RoS subsystem to provide GPIO support. Add
GPIO device entry and also add GPIO signalled ACPI event template for
reference.

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