feat(rdn2cfg3): add tftf support

Add tftf support for Neoverse Reference Design RD-N2-Cfg3 platform. It
is a variant of RD-N2 platform with a 10x6 mesh. For this platform, the
PLATFORM_VARIANT macro should be set to 3 at compile time.

Signed-off-by: Tony K Nadackal <tony.nadackal@arm.com>
Change-Id: I67cd441586c1e437d816cedd086d55f2b5c85718