macintosh/via-pmu68k: Don't load driver on unsupported hardware

Don't load the via-pmu68k driver on early PowerBooks. The M50753 PMU
device found in those models was never supported by this driver.
Attempting to load the driver usually causes a boot hang.

Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Reviewed-by: Michael Schmitz <schmitzmic@gmail.com>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>