SecurityPkg: Change default value source

SecurityPkg: Change default value source

https://bugzilla.tianocore.org/show_bug.cgi?id=2713
In current code, If TCG2_PHYSICAL_PRESENCE_FLAGS_VARIABLE variable
is not exist, code will get default value from two places.
This fix is to make the default value comes from the PCD
gEfiSecurityPkgTokenSpaceGuid.PcdTcg2PhysicalPresenceFlags

Signed-off-by: Maggie Chu <maggie.chu@intel.com>
Cc: Eric Dong <eric.dong@intel.com>
Cc: Jian J Wang <jian.j.wang@intel.com>
Cc: Chao Zhang <chao.b.zhang@intel.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>