DynamicTablesPkg: Fix Boot arch flag width

DynamicTablesPkg: Fix Boot arch flag width

The ArmBootArch field of the FADT table is 16-bit wide. The
VS2017 compiler reports 'warning C4244: '=': conversion from
'UINT32' to 'UINT16', possible loss of data' when assigning the
CM_ARM_BOOT_ARCH_INFO.BootArchFlags value as the width of this
field in CM_ARM_BOOT_ARCH_INFO is 32-bit wide.

To fix this warning, update the CM_ARM_BOOT_ARCH_INFO struct
to make the  BootArchFlags field 16-bit wide. This also makes
it compatible with the ACPI FADT specification.

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