configs/rdn2: add SCP_PLATFORM_VARIANT build flag for rdn2 platform

SCP_PLATFORM_VARIANT is a product specific build parameter passed using
the build option PRODUCT_BUILD_PARAMS. It is used to configure a
particular variant of a product. The default variant of RD-N2 platform
has id 0. Provide the same as SCP_PLATFORM_VARIANT.

Signed-off-by: Aditya Angadi <aditya.angadi@arm.com>
Change-Id: I2a1624d0759f525c9bb9821f2f3434d442810b3b