MdePkg/IndustryStandard/Ipmi: Use union for bitmap fields

MdePkg/IndustryStandard/Ipmi: Use union for bitmap fields

This commit enhances the bitmap fields defined in the IPMI header files,
union types will be used to provide the users with both the individual
bitmap access and the whole byte/word access.

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