serial: mxs: spl: Remove empty serial_* functions from SPL code

Remove the empty bodies from serial_* functions from MXS SPL code.
These empty implementations are now in common/serial.c instead so
declaring them also in the SPL code would cause a colision once
serial multi is enabled unconditionally.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Marek Vasut <marek.vasut@gmail.com>
Cc: Tom Rini <trini@ti.com>
Cc: Stefano Babic <sbabic@denx.de>

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

@@ -124,10 +124,6 @@ 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)
 {