ARM: ixp4xx_defconfig: add CONFIG_SERIAL_OF_PLATFORM

My ixp42x-welltech-epbx100 does not print anything on console without
CONFIG_SERIAL_OF_PLATFORM.

Link: https://lore.kernel.org/r/20210504193457.4008384-2-clabbe@baylibre.comSigned-off-by: Corentin Labbe <clabbe@baylibre.com>
Signed-off-by: Olof Johansson <olof@lixom.net>

arch/arm/configs/ixp4xx_defconfig

@@ -152,6 +152,7 @@ CONFIG_INPUT_IXP4XX_BEEPER=y
 CONFIG_SERIAL_8250=y
 CONFIG_SERIAL_8250_CONSOLE=y
 CONFIG_SERIAL_8250_NR_UARTS=2
+CONFIG_SERIAL_OF_PLATFORM=y
 CONFIG_I2C=y
 CONFIG_I2C_CHARDEV=y
 CONFIG_SENSORS_W83781D=y