Silicon/AMD/Styx: move SOC version macros to common header

Instead of adding yet another redefinition in the next patch, move
the silicon revision testing macros into a shared header file.

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