xen/arm: types: Specify the zero padding in the definition of PRIregister

xen/arm: types: Specify the zero padding in the definition of PRIregister

The definition of PRIregister varies between Arm32 and Arm64 (32-bit vs
64-bit). However, some of the users uses the wrong padding and others
are not using padding at all.

For more consistency, the padding is now moved into the PRIregister and
varies depending on the architecture.

Signed-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Volodymyr Babchuk <volodymyr_babchuk@epam.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>