[AVR32] Make sure all genclocks have a parent

Initialize the parent field of each generic clock by looking at the
PM registers. This means that the genclock operations can always
assume that the parent field is non-null, so they don't have to
check. Also remove a few unnecessary BUG_ON()s.

Extracted from a patch by David Brownell.

Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>