This adds the ability for the driver to access UARTs via MMIO instead
of PIO selectively at runtime, and exposes a new function to add an
MMIO port.
In an ideal world, MMIO accessors would be generic and have architecture
specific memory barriers. However, existing drivers don't have them and
most of those "bare metal" drivers tend to be for x86 which doesn't need
them. If necessary, those can be added later.
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
9fb22d0478