MinPlatformPkg: Add PCDs for Serial Terminal feature

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=3014



Add PcdSerialTerminalEnable to enable/disable Serial Terminal feature,
this feature supports console redirect after the shell is loaded.

Cc: Eric Dong <eric.dong@intel.com>
Cc: Chasel Chiu <chasel.chiu@intel.com>
Cc: Nate DeSimone <nathaniel.l.desimone@intel.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Signed-off-by: Heng Luo <heng.luo@intel.com>
Reviewed-by: Nate DeSimone <nathaniel.l.desimone@intel.com>