tc2: Add support for TC2_VAR_EXPERIMENT_POWER

Similarly to what's been done for TC0, an experimental variant for TC2
is added.
This new variant option will add software features like mpmm, tcop,
thermal management to the platform so developers can have a software
reference for experimenting the above components.

To enable the experimental features, please add the following in your
build command:

EXTRA_CONFIG_ARGS+=-DSCP_PLATFORM_VARIANT=1

The standard build still falls into the traditional set of software
support.

Signed-off-by: Nicola Mazzucato <nicola.mazzucato@arm.com>
Change-Id: I8354480822bbb7aa223beaaa8dd44573bab6a1dd