ShellPkg SmbiosView: Add missing decoding of SlotType AGP8X

ShellPkg SmbiosView: Add missing decoding of SlotType AGP8X

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=344

SlotType AGP8X was added in SMBIOS spec 2.3.4, but the decoding
of it is missing. I found it when I am adding SMBIOS spec 3.1.0
support.

Cc: Jaben Carsey <jaben.carsey@intel.com>
Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>