macintosh/via-pmu68k: Initialize PMU driver with setup_arch and arch_initcall

The PMU watchdog will power down the system if the kernel is slow
to start up, e.g. due to unpacking a large initrd. The powerpc
version of this driver (via-pmu.c) has a solution for the same
problem. It uses this call sequence:

setup_arch
	find_via_pmu
		init_pmu
...
arch_initcall
	via_pmu_start

Bring via-pmu68k.c into line with via-pmu.c to fix this issue.

Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Tested-by: Stan Johnson <userm57@yahoo.com>
Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>