serial: ns16550: Support ns16550 compatible pci uart devices

There are many pci uart devices which are ns16550 compatible. We can
describe them in the board dts file and use it as the U-Boot serial
console as specified in the chosen node 'stdout-path' property.

Those pci uart devices can have their register be memory-mapped, or
i/o-mapped. The driver will try to use the memory-mapped register if
the reg property in the node has an entry to describe the memory-mapped
register, otherwise i/o-mapped register will be used.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Simon Glass <sjg@chromium.org>