Create configuration option for restricted ns16550 functions

This patch will create a configuration option for a minimum configuration for
the ns16550 serial driver at drivers/serial/ns16550.c and will apply this new
configuration option to the SIMPC8313.h config file in order to fix the NAND
bootstrap build error.  This option will exclude all functions with exception of
NS16550_putc and NS16550_init.  This will be used primarily to save space and
remove unused code from builds in which space is limited.

Signed-off-by: Ron Madrid <ron_madrid@sbcglobal.net>