serial/ns16550: don't build serial_ns16550 with MIN_FUNCTIONS

serial/ns16550: don't build serial_ns16550 with MIN_FUNCTIONS

CONFIG_NS16550_MIN_FUNCTIONS is used by small SPLs to gain access to basic
ns16550 output code without pulling in things not needed by the SPL.

This previously only worked with non-MULTI configs.  Recently MULTI was
made mandatory, and MIN_FUNCTIONS fails like this:

drivers/serial/libserial.o: In function `calc_divisor.clone.0':
serial_ns16550.c:(.text.calc_divisor.clone.0+0x24): undefined reference to `get_bus_freq'
drivers/serial/libserial.o: In function `_serial_getc':
(.text._serial_getc+0x30): undefined reference to `NS16550_getc'
drivers/serial/libserial.o: In function `_serial_tstc':
(.text._serial_tstc+0x30): undefined reference to `NS16550_tstc'
drivers/serial/libserial.o: In function `_serial_setbrg':
(.text._serial_setbrg+0x3c): undefined reference to `NS16550_reinit'
make[1]: *** [/tmp/u-boot/spl/u-boot-spl] Error 1
make: *** [/tmp/u-boot/spl/u-boot-spl.bin] Error 2

With MIN_FUNCTIONS we don't need anything from this file, so don't build
it.  The conditional needs to be in the file itself rather than the
makefile, because the config symbols are only imported to the makefiles
once, not separately for the SPL phase of the build.

Signed-off-by: Scott Wood <scottwood@freescale.com>