xen/arm: io: Extend write/read handler to pass the register in parameter

xen/arm: io: Extend write/read handler to pass the register in parameter

Rather than letting each handler to retrieve the register used by the
I/O access, add a new parameter to pass the register in parameter.

This will help to implement generic register manipulation on I/O access
such as sign-extension and endianess.

Read handlers need to modify the value of the register, so a pointer to
it is given in argument. Write handlers shouldn't modify the register,
therefore only a plain value is given.

Signed-off-by: Julien Grall <julien.grall@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>