macintosh/via-pmu: Explicitly specify CONFIG_PPC_PMAC dependencies

At present, CONFIG_ADB_PMU depends on CONFIG_PPC_PMAC. When this gets
relaxed to CONFIG_PPC_PMAC || CONFIG_MAC, those Kconfig symbols with
implicit deps on PPC_PMAC will need explicit deps. Add them now.
No functional change.

Tested-by: Stan Johnson <userm57@yahoo.com>
Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>