serial: ns16550: Support run-time configuration

At present this driver uses an assortment of CONFIG options to control
how it accesses the hardware. This is painful for platforms that are
supposed to be controlled by a device tree or a previous-stage bootloader.

Add a new CONFIG option to enable fully dynamic configuration. This
controls register spacing, size, offset and endianness.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Aiden Park <aiden.park@intel.com>
Tested-by: Aiden Park <aiden.park@intel.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
[bmeng: squashed in http://patchwork.ozlabs.org/patch/1232929/

]
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>