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.com


Signed-off-by: Corentin Labbe <clabbe@baylibre.com>
Signed-off-by: Olof Johansson <olof@lixom.net>