ARM: exynos: Disable serial support in SPL

ARM: exynos: Disable serial support in SPL

The exynos5 platforms use DM in U-Boot and do not use DM in SPL. The serial
driver, serial_s5p.c, is DM-only. This is OK for U-Boot, but in SPL, this
will fail with the following compile error:

drivers/built-in.o: In function `get_current':
...u-boot/drivers/serial/serial.c:387: undefined reference to `default_serial_console'

This warning happens because common/console.c is compiled into U-Boot SPL
if CONFIG_SPL_SERIAL_SUPPORT . The common/console.c invokes serial_*()
functions and since exynos5 does not use DM in SPL, these functions come
from drivers/serial/serial.c . The serial_*() locate default serial port
by calling default_serial_console(), but because the serial_s5p.c is DM-only,
it does no longer define default_serial_console(). Thus the error.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Simon Glass <sjg@chromium.org>
Cc: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>