xen/arm: io: Abstract sign-extension

xen/arm: io: Abstract sign-extension

In order to avoid code duplication (both handle_read() and
handle_ioserv() contain the same code for the sign-extension)
put this code to a common helper to be used for both.

Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
CC: Julien Grall <julien.grall@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Acked-by: Julien Grall <jgrall@amazon.com>
[On Arm only]
Tested-by: Wei Chen <Wei.Chen@arm.com>