Platform/Sgi: Add GED support

Add ACPI Generic Event Device (GED) support for Arm's reference design
platforms. The SP804 dual-timer interrupt is used as the event source
for GED.

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>