IntelSiliconPkg/IntelSiliconPkg.dsc: Add a driver to build

IntelSiliconPkg/IntelSiliconPkg.dsc: Add a driver to build

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=1647

Add a driver MicrocodeFlashAccessLibNull.inf to the
[Components] section to make sure it would build correctly.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Zhichao Gao <zhichao.gao@intel.com>
Cc: Ray Ni <ray.ni@intel.com>
Cc: Rangasai V Chaganty <rangasai.v.chaganty@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Reviewed-by: Rangasai V Chaganty <rangasai.v.chaganty@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>