m68k: move coldfire MMU initialization code

The M54[78]x ColdFire parts are not the only members of the ColdFire family
that have an MMU. But currently some of the early MMU initialization code
is inside the startup code specific to only the ColdFire M54[78]x parts.
Move that early ColdFire MMU init code so that it is run for other ColdFire
parts running with MMU enabled.

Specifically this means that the MMU initialization code will now also be
run for the ColdFire M5441x parts when running with MMU enabled.

The code move meant that the extern definition for the mmu_context_init()
function had to be moved as well. To make it clear that is ColdFire specific
I have renamed that with a "cf_" in front of it and put its extern definition
in the mcfmmu.h (which is already included by the setup code).

Reported-by: Angelo Dureghello <angelo@sysam.it>
Tested-by: Angelo Dureghello <angelo@sysam.it>
Signed-off-by: Greg Ungerer <gerg@linux-m68k.org>