ns16550: change to allow 32 bit access to registers

If CONFIG_SYS_NS16550_MEM32 is defined then 32 bit memory
mapped access will be used to read/write the uart registers.

This is especially useful for SoC devices that implement 16550
compatible uarts but that have peripheral access width constraints.

Signed-off-by: Dave Aldridge <fovsoft@gmail.com>