ns16550: change map_sysmem to map_physmem

ns16550: change map_sysmem to map_physmem

Change map_sysmem() to map_physmem(,,MAP_NOCACHE). Though map_sysmem()
can be used to map system memory, it might be wrong to use it for I/O
ports.  The map_physmem() serves the same purpose to translate physical
address to virtual address with the additional flag to take care of cache
property. Most drivers use map_physmem() since I/O ports access should be
uncached. As ns16550 is a driver, it should use map_physmem() rather
than map_sysmem().

Signed-off-by: Thomas Chou <thomas@wytron.com.tw>
Acked-by: Simon Glass <sjg@chromium.org>