i.MX28: Avoid redefining serial_put[cs]()

Do not define serial_putc() and serial_puts() calls if
CONFIG_SPL_SERIAL_SUPPORT is set.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Detlev Zundel <dzu@denx.de>
Cc: Fabio Estevam <fabio.estevam@freescale.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Wolfgang Denk <wd@denx.de>
Acked-by: Stefano Babic <sbabic@denx.de>