xen/arm: io: Shorten the name of the fields and clean up

xen/arm: io: Shorten the name of the fields and clean up

The field names in the IO emulation are really long and use repeatedly
the term handler which make some line cumbersome to read:

mmio_handler->mmio_handler_ops->write_handler

Also take the opportunity to do some clean up:
    - Avoid "handler" vs "handle" in register_mmio_handler
    - Use a local variable to initialize handler in
    register_mmio_handler
    - Add a comment explaining the dsb(ish) in register_mmio_handler
    - Rename the structure io_handler into vmmio because the io_handler
    is in fine handling multiple handlers and the name a the fields was
    io_handlers. Also rename the field io_handlers to vmmio
    - Rename the field mmio_handler_ops to ops because we are in the
    structure mmio_handler to not need to repeat it
    - Rename the field mmio_handlers to handlers because we are in the
    vmmio structure
    - Make it clear that register_mmio_ops is taking an ops and not an
    handle
    - Clean up local variable to help to understand the code

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