feat(fvp): support building RSS comms driver

feat(fvp): support building RSS comms driver

On one hand, there is currently no upstream platform supporting the
RSS. On the other hand, we are gradually introducing driver code for
RSS. Even though we cannot test this code in the TF-A CI right now, we
can at least build it to make sure no build regressions are introduced
as we continue development.

This patch adds support for overriding PLAT_RSS_NOT_SUPPORTED build
flag (which defaults to 1 on the Base AEM FVP) from the command
line. This allows introducing an ad-hoc CI build config with
PLAT_RSS_NOT_SUPPORTED=0, which will correctly pull in the RSS and MHU
source files. Of course, the resulting firmware will not be
functional.

Change-Id: I2b0e8dd03bf301e7063dd4734ea5266b73265be1
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>