Silicon/SynQuacer: make lowest supported f/w version PCD configurable

Allow the lowest supported firmware version to be configured by PCD
so that each platform can set it individually.

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