MdePkg/IndustryStandard: Add header file for SPMI ACPI table

MdePkg/IndustryStandard: Add header file for SPMI ACPI table

REF:https://bugzilla.tianocore.org/show_bug.cgi?id=840

Add the header file for Service Processor Management Interface ACPI table
definition.

Cc: Younas Khan <pmdyounaskhan786@gmail.com>
Cc: Michael 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: Jiewen Yao <jiewen.yao@intel.com>