clk: at91: allow setting all PMC clock parents via DT

We need to have clocks accessible via phandle to select them
as peripheral clock parent using assigned-clock-parents in DT.
Add support for PLLACK/PLLBCK/AUDIOPLLCK clocks where available.

Signed-off-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Acked-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lkml.kernel.org/r/fa39cc10dab8341ea4bc2b7152be9217b2cd34a5.1588630999.git.mirq-linux@rere.qmqm.pl


Signed-off-by: Stephen Boyd <sboyd@kernel.org>