Silicon/AMD/Styx/StyxDtbLoaderLib: replace DO_KCS macro reference with PCD

Get rid of the CPP conditionals, and use the new boolean PCD for KCS
support instead.

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