Documentation: dt: add bindings for keystone pll control controller

The main pll controller used to drive theC66x CorePacs, the switch fabric,
and a majority of the peripheral clocks (all but the ARM CorePacs, DDR3 and
the NETCP modules) requires a PLL Controller to manage the various clock
divisions, gating, and synchronization.

Reviewed-by: Arnd Bergmann <[e-mail]>
Acked-by: Mike Turquette <[e-mail]>
Signed-off-by: Ivan Khoronzhuk <[e-mail]>
[[e-mail]: Fixed the subject line]
Signed-off-by: Santosh Shilimkar <[e-mail]>

Documentation/devicetree/bindings/clock/ti-keystone-pllctrl.txt

···
+* Device tree bindings for Texas Instruments keystone pll controller
+
+The main pll controller used to drive theC66x CorePacs, the switch fabric,
+and a majority of the peripheral clocks (all but the ARM CorePacs, DDR3 and
+the NETCP modules) requires a PLL Controller to manage the various clock
+divisions, gating, and synchronization.
+
+Required properties:
+
+- compatible:		"ti,keystone-pllctrl", "syscon"
+
+- reg:			contains offset/length value for pll controller
+			registers space.
+
+Example:
+
+pllctrl: pll-controller@0x02310000 {
+	compatible = "ti,keystone-pllctrl", "syscon";
+	reg = <0x02310000 0x200>;
+};