ARM: ixp4xx: don't select SERIAL_OF_PLATFORM

Platforms should not normally select all the device drivers, leave that
up to the user and the defconfig file.

In this case, we get a warning for randconfig builds:

WARNING: unmet direct dependencies detected for SERIAL_OF_PLATFORM
  Depends on [n]: TTY [=y] && HAS_IOMEM [=y] && SERIAL_8250 [=n] && OF [=y]
  Selected by [y]:
  - MACH_IXP4XX_OF [=y] && ARCH_IXP4XX [=y]

Fixes: 9540724c ("ARM: ixp4xx: Add device tree boot support")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Olof Johansson <olof@lixom.net>