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>

arch/arm/cpu/arm926ejs/mx28/spl_boot.c

@@ -123,8 +123,10 @@ inline void board_init_r(gd_t *id, ulong dest_addr)
 		;
 }
 
+#ifndef CONFIG_SPL_SERIAL_SUPPORT
 void serial_putc(const char c) {}
 void serial_puts(const char *s) {}
+#endif
 void hang(void) __attribute__ ((noreturn));
 void hang(void)
 {