IntelFsp2Pkg/FspApi.h: Add comment for structure definition.

Add doxygen style comment for structure definition.

Cc: Giri P Mudusuru <giri.p.mudusuru@intel.com>
Cc: Satya P Yarlagadda <satya.p.yarlagadda@intel.com>
Cc: Maurice Ma <maurice.ma@intel.com>
Cc: Ravi P Rangarajan <ravi.p.rangarajan@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jiewen Yao <jiewen.yao@intel.com>
Reviewed-by: Giri P Mudusuru <giri.p.mudusuru@intel.com>