spl: ti: Avoid serial calls when serial support is disabled

If CONFIG_SPL_SERIAL_SUPPORT is not set, then the build will fail:

board/ti/am335x/built-in.o: In function `spl_start_uboot':
board/ti/am335x/board.c:247: undefined reference to `serial_tstc'
board/ti/am335x/board.c:247: undefined reference to `serial_getc'

Avoid the calls to the serial functions in that case.

Signed-off-by: Alex Kiernan <alex.kiernan@gmail.com>