DynamicTablesPkg: Fix Proc node length assignment

DynamicTablesPkg: Fix Proc node length assignment

The length field for the Processor Hierarchy node structure is
8-bit wide while the number of private resource field is 32-bit
wide. Therefore, the GetProcHierarchyNodeSize() returns the size
as a 32-bit value.

The VS2017 compiler reports 'warning C4244: '=': conversion from
'UINT32' to 'UINT8', possible loss of data' while assigning the
length field of the Processor Hierarchy node structure.

To fix this, a type cast is added. In addition, there is a check
to ensure that the Processor Hierarchy node size does not exceed
MAX_UINT8.

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