Silicon/AMD/Styx: introduce boolean PCD for KCS/IPMI support

In order to get rid of another CPP macro, introduce a boolean PCD
that we will set in the platform to enable IPMI/KCS support.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>