scp: add build option to configure the product variant

Currently, there are 3 product variants in SCP firmware for TC2.
1. SCP_PLATFORM_VARIANT_STD
2. SCP_PLATFORM_VARIANT_EXPERIMENT - Standard variant + experimental
   power features (MPMM, traffic cop, thermal management)
3. SCP_PLATFORM_VARIANT_MPMM - Standard variant + MPMM

Signed-off-by: Davidson K <davidson.kumaresan@arm.com>