m68knommu: create and use a common M53xx ColdFire class of CPUs

The current CONFIG_M532x support definitions are actually common to a larger
set of version 3 ColdFire CPU types. In the future we want to add support for
the 537x family. It is very similar to the 532x internally, and will be able
to use most of the same definitions.

Create a CONFIG_M53xx option that is enabled to support any of the common
532x and 537x CPU types. Convert the current users of CONFIG_M532x to use
CONFIG_M53xx instead.

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