[uart] Support 16550 UARTs accessed via either MMIO or port I/O

Use the combined accessors ioread8() and iowrite8() to read and write
16550 UART registers, to allow the decision between using MMIO and
port I/O to be made at runtime.

Minimise the increase in code size for x86 by ignoring the register
shift, since this is essentially used only for non-x86 SoCs.

Signed-off-by: Michael Brown <mcb30@ipxe.org>