cuda/pmu: Make find_via_cuda/pmu init functions

Make `find_via_cuda` and `find_via_pmu` initialization functions.
Previously, their definitions in `drivers/macintosh/via-cuda.h` include
the `__init` attribute but their alternative definitions in
`arch/powerpc/powermac/sectup./c` and prototypes in `include/linux/
cuda.h` and `include/linux/pmu.h` do not use the `__init` macro. Since,
only initialization functions call `find_via_cuda` and `find_via_pmu`
it is safe to label these functions with `__init`.

Signed-off-by: Nick Child <nick.child@ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20211216220035.605465-21-nick.child@ibm.com