clk: at91: allow setting PCKx parent via DT

This exposes PROGx clocks for use in assigned-clocks DeviceTree property
for selecting PCKx parent clock.

Signed-off-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Link: https://lkml.kernel.org/r/0054532c00163ddf405dad658b32f0d7d97fcc8e.1588630999.git.mirq-linux@rere.qmqm.pl


Acked-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>