dt-bindings: soc: amlogic: document System Control registers

Document the System Control registers regions found on all Amlogic
SoC families and it's clock, power, pinctrl and phy subnodes.

The regions has various independent registers tied to other
hardware devices, thus the syscon compatible.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>