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>

Documentation/devicetree/bindings/arm/atmel-pmc.txt

-* Power Management Controller (PMC)
-
-Required properties:
-- compatible: Should be "atmel,<chip>-pmc".
-	<chip> can be: at91rm9200, at91sam9260, at91sam9g45, at91sam9n12,
-	at91sam9x5, sama5d3
-
-- reg: Should contain PMC registers location and length
-
-Examples:
-	pmc: pmc@fffffc00 {
-		compatible = "atmel,at91rm9200-pmc";
-		reg = <0xfffffc00 0x100>;
-	};