Platform/Sgi: Define RD-N2 platform id values

Add RD-N2 platform identification values including the part number
and configuration number. This information will be used in populating
the SMBIOS tables.

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>