MdePkg: Add flags and MinTransferSize to Generic Initiator

MdePkg: Add flags and MinTransferSize to Generic Initiator

Bugzilla: 3516 (https://bugzilla.tianocore.org/show_bug.cgi?id=3516)

Make changes to ACPI 6.4 header according to the latest specification:
- ACPI 6.4 January 2021, Table 5.59, Section 5.2.27.1 & Section 5.2.27.4
- Mantis ID 1991 (https://mantis.uefi.org/mantis/view.php?id=1991)

Signed-off-by: Chris Jones <christopher.jones@arm.com>
Reviewed-by: Liming Gao <gaoliming@byosoft.com.cn>
Reviewed-by: Sami Mujawar <sami.mujawar@arm.com>