xen/arm: io: Protect the handlers with a read-write lock

xen/arm: io: Protect the handlers with a read-write lock

Currently, accessing the I/O handlers does not require to take a lock
because new handlers are always added at the end of the array. In a
follow-up patch, this array will be sort to optimize the look up.

Given that most of the time the I/O handlers will not be modify,
using a spinlock will add contention when multiple vCPU are accessing
the emulated MMIOs. So use a read-write lock to protected the handlers.

Finally, take the opportunity to re-indent correctly domain_io_init.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>