product/rdn2: add support for variant 3 of rdn2 platform

Add support for variant 3 of RD-N2 platform. It is a variant of RD-N2
platform with a 10x6 mesh. For this variant, the RD-N2 platform support
is reused using the compile time macro PLATFORM_VARIANT. 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>
Signed-off-by: Vijayenthiran Subramaniam <vijayenthiran.subramaniam@arm.com>
Change-Id: I53c4feeba066d961bd6131c8f7a0e3868d4bf1a7