Build: Update RD-V3 to updated structure for neoverse_rd

RD-V3 is now a platform available under the neoverse_rd. Add
support for RD-V3-Cfg1 and RD-V3-Cfg2 through setting the
value of TFM_PLATFORM_VARIANT in case it is required.

Signed-off-by: Antonio de Angelis <antonio.deangelis@arm.com>
Change-Id: I4cf6423e1b1921ebaa5c6406092dedf459473ee9
2 files changed