ARM: kirkwood: fix DT building and update defconfig

commit

  844703c1

 ARM: kirkwood: Remove all remaining trace of DNS-320/325 platform code

removed the last kirkwood DT/platform board in kirkwood_defconfig, this
triggered a rescan of the config symbols, the result of which breaks the
build because KIRKWOOD_DT does not select OF_IRQ.

This commit fixes the regression and updates the defconfig to
explicitly enable SERIAL_OF_PLATFORM and KIRWOOD_DT.

Signed-off-by: Jason Cooper <jason@lakedaemon.net>