ns16550: limit mapped MMIO size

ns16550: limit mapped MMIO size

There's no point in mapping more than the memory we actually may need
to touch, and in fact the too large region could actually extend into
another device's one (which currently is benign on x86 since only a
single page gets mapped anyway, but which is a latent bug on ARM
whenever PCI support gets enabled there).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>