MdeModulePkg: Correct memory type in PrePiDxeCis.h

MdeModulePkg: Correct memory type in PrePiDxeCis.h

The enumeration in MdePkg/Include/Pi/PiDxeCis.h has a duplicated entry,
so the 8th position in the list doesn't count as index 7. The value
EfiGcdMemoryTypeUnaccepted will have when added before
EfiGcdMemoryTypeMaximum will be 6.

Cc: Min M Xu <min.m.xu@intel.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Signed-off-by: Dionna Glaze <dionnaglaze@google.com>
Reviewed-by: Min Xu <min.m.xu@intel.com>
Reviewed-by: Liming Gao <gaoliming@byosoft.com.cn>