arm/arm64: mmu: Stop mapping an assumed IO region

By providing a proper ioremap function, we can just rely on devices
calling it for each region they need (as they already do) instead of
mapping a big assumed I/O range. We don't require the MMU to be
enabled at the time of the ioremap. In that case, we add the mapping
to the identity map anyway. This allows us to call setup_vm after
io_init. Why don't we just call setup_vm before io_init, I hear you
ask? Well, that's because tests like sieve want to start with the MMU
off, later call setup_vm, and all the while have working I/O. Some
unit tests are just really demanding...

While at it, ensure we map the I/O regions with XN (execute never),
as suggested by Alexandru Elisei.

Reviewed-by: Alexandru Elisei <alexandru.elisei@arm.com>
Tested-by: Alexandru Elisei <alexandru.elisei@arm.com>
Signed-off-by: Andrew Jones <drjones@redhat.com>