MdeModulePkg PCD: Allow SkuId to be changed only once

MdeModulePkg PCD: Allow SkuId to be changed only once

Current PI spec has no clear description about whether the
SkuId could be changed multiple times or not during one boot.

If the SkuId could be changed multiple times during one boot,
different modules may get inconsistent PCD values.
And DynamicHii PCD maps to UEFI variable, once one DynamicHii
PCD(UEFI variable) is set for one SkuId, then the PCD value
will be always from UEFI variable but not PCD database, even
the SkuId is set to other value.

This patch is to update PCD drivers to allow SkuId to be
changed only once during one boot.

Cc: Liming Gao <liming.gao@intel.com>
Cc: Michael Kinney <michael.d.kinney@intel.com>
Cc: Feng Tian <feng.tian@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>