It is unwise to use sources other than the oscillator and PLLD_PER for
the PCM peripheral (and perhaps others - TBD) because their rate can
change and they may even be switched off, so explicitly restrict the
choice using dummy entries in the list of potential parents (item index
is significant).

See: https://github.com/raspberrypi/linux/issues/1949

Signed-off-by: Phil Elwell <phil@raspberrypi.org>