DynamicTablesPkg: Fix GT Block length assignment

DynamicTablesPkg: Fix GT Block length assignment

The VS2017 compiler reports 'warning C4267: '=': conversion from
'size_t' to 'UINT16', possible loss of data'.

The sizeof() operator is used to calculate the size of the
GT Block structure. The length field in the GT Block structure
is 16-bit wide. Since the return type of sizeof() operator
is size_t the VS2017 compiler reports the above warning.

To fix the warning, an explicit type cast is added. An additional
check is also performed to ensure that the calculated GT Block
length does not exceed MAX_UINT16.

Signed-off-by: Sami Mujawar <sami.mujawar@arm.com>
Reviewed-by: Alexei Fedorov <Alexei.Fedorov@arm.com>
Reviewed-by: Philippe Mathieu-Daude <philmd@redhat.com>