Platform/Sgi: Fix interrupt numbers for RD-N2

The generic timer and watchdog timer interrupt numbers on the RD-N2
platform is different than those on the other SGI/RD platforms. So in
order to reuse the existing GTDT ACPI table for all the supported
platforms including RD-N2, use the interrupt values as defined by PCD in
the GTDT ACPI table.

Change-Id: Ibf7fb31fc60424cb6500f7a7d0babf43b6807bfa
Signed-off-by: Pranav Madhu <pranav.madhu@arm.com>