IntelSiliconPkg: Updated IgdOpregion.h based on latest spec

IntelSiliconPkg: Updated IgdOpregion.h based on latest spec

Updated IgdOpregion.h to align with latest specification
https://01.org/sites/default/files/documentation/skl_opregion_rev0p5.pdf

1) Updated Mailbox structures to align with latest spec
2) Added Mailbox 5 structure
3) Added defines for Signature and Mailbox support

Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>
Reviewed-by: Maurice Ma <maurice.ma@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Giri P Mudusuru <giri.p.mudusuru@intel.com>