dt-bindings: clock: meson8b: add the clock inputs

The clock controller on Meson8/Meson8b/Meson8m2 has three (known)
inputs:
- "xtal": the main 24MHz crystal
- "ddr_pll": some of the audio clocks use the output of the DDR PLL as
input
- "clk_32k": an optional clock signal which can be connected to GPIOAO_6
(which then has to be switched to the CLK_32K_IN function)

Add the inputs to the documentation so we can wire up these inputs in a
follow-up patch.

Reviewed-by: Rob Herring <[e-mail]>
Signed-off-by: Martin Blumenstingl <[e-mail]>
Signed-off-by: Jerome Brunet <[e-mail]>

Documentation/devicetree/bindings/clock/amlogic,meson8b-clkc.txt

···
 	- "amlogic,meson8m2-clkc" for Meson8m2 (S812) SoCs
 - #clock-cells: should be 1.
 - #reset-cells: should be 1.
+- clocks: list of clock phandles, one for each entry in clock-names
+- clock-names: should contain the following:
+  * "xtal": the 24MHz system oscillator
+  * "ddr_pll": the DDR PLL clock
+  * "clk_32k": (if present) the 32kHz clock signal from GPIOAO_6 (CLK_32K_IN)
 
 Parent node should have the following properties :
 - compatible: "amlogic,meson-hhi-sysctrl", "simple-mfd", "syscon"