m68k: coldfire: enable devicetree use

With minimal devicetree support in place enable the devicetree kernel
configuration options to use it on ColdFire platforms.

This is purely a proof-of-concept change. It is enough to use basic
devicetrees on the MCF5208EVB and M5475EVB platforms.

Signed-off-by: Greg Ungerer <gerg@linux-m68k.org>