MdePkg/DMAR: Add the definition for DMA_CTRL_PLATFORM_OPT_IN_FLAG bit

MdePkg/DMAR: Add the definition for DMA_CTRL_PLATFORM_OPT_IN_FLAG bit

For the support of VTd 2.5, add the BIT definition of
DMA_CTRL_PLATFORM_OPT_IN_FLAG

Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Hao Wu <hao.a.wu@intel.com>
Reviewed-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>