MdePkg ACPI65: Update MADT Revision pre ACPI Spec 6.5

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=4474



ACPI_Spec_6_5_Aug29 Table 5.19 page 128 that MADT Revision
field is 6.

Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Cc: Zhiguang Liu <zhiguang.liu@intel.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Signed-off-by: Aryeh Chen <aryeh.chen@intel.com>
Reviewed-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Liming Gao <gaoliming@byosoft.com.cn>
Tested-by: Aryeh Chen <aryeh.chen@intel.com>