plat/arm/sgi: add platform id value for rdn2 platform

In preparation for adding the board support for RD-N2 platform, add
macros to define the platform id and the corresponding SCMI platform
info for the RD-N2 platform.

Change-Id: Ie764ae618732b39e316f7ed080421f5d79adab21
Signed-off-by: Aditya Angadi <aditya.angadi@arm.com>
2 files changed