m68k/mvme16x: Adopt common boot console

In a multi-platform kernel binary we only need one early console instance.

The difficulty here is that the common early console is started by
early_param(), whereas the MVME16x instance is started later by
config_mvme16x(). That means some interrupt setup must be done earlier.

Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Tested-by: Stephen N Chivers <schivers@csc.com.au>
[Geert] Tag debug_cons_write() with __ref to kill section mismatch warning
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>