DynamicTablesPkg: Set EndTag's Checksum if RdList is modified

DynamicTablesPkg: Set EndTag's Checksum if RdList is modified

ACPI 6.4, s6.4.2.9 "End Tag":
"This checksum is generated such that adding it to the sum of all the data
bytes will produce a zero sum."
"If the checksum field is zero, the resource data is treated as if the
checksum operation succeeded. Configuration proceeds normally."

To avoid re-computing checksums, if a new resource data elements is
added/removed/modified in a list of resource data elements, the AmlLib
resets the checksum to 0.

This patch also refactors the AmlAppendRdNode() function by getting the
last Resource Data node directly instead of iterating over all the
elements of the list of Resource Data node.

Reviewed-by: Sami Mujawar <sami.mujawar@arm.com>
Signed-off-by: Pierre Gondois <Pierre.Gondois@arm.com>