clk: at91: modernize sckc binding

Remove the need for child nodes in the sckc binding and register the whole
sckc tree (3 clocks in total) from the sckc node.

DT backward compatibility is kept by looking for properties in child nodes
when they are not present in the sckc node.

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