dm: ns16550: Don't map_physmem for I/O ports

If the UART is to be accessed using I/O port accessors (inb & outb) then
using map_physmem doesn't make sense, since it operates in a different
memory space. Remove the call to map_physmem when
CONFIG_SYS_NS16550_PORT_MAPPED is defined, allowing I/O port addresses
to not be mangled by the incorrect mapping.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>