dt-bindings: clk: at91: Document new PMC binding

Document the new PMC binding with only one PMC node for all the PMC clocks
instead of one node per clock as this proved to be problematic.

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