dt-bindings: arm: remove PMC bindings

The PMC bindings are fully described in
Documentation/devicetree/bindings/clock/at91-clock.txt. Remove the
duplicate and incomplete documentation.

Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>