Platform/Sgi: Macro definitions for ACPI _OSC

Add helper macros required for use with ACPI Operating System
Capabilities (_OSC) control method. The macros for capability DWORD and
return status value DWORD are defined.

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>