Silicon/AMD/Styx/PlatInitPei: replace XGBE CPP conditional with PCD

We already have a boolean PCD that tells us whether XGBE support should
be enabled, so use it instead of C preprocessor conditionals.

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