Documentation: dt: add bindings for rockchip-cpufreq

Documentation: dt: add bindings for rockchip-cpufreq

Add the device tree bindings document for ROCKCHIP CPUFreq driver.
The operating-points-v2 binding allows us to provide an opp-supported-hw
property for each OPP to define when it is available and an
opp-microvolt-<name> property to choose a suitable voltage for OPP.

This driver reads SoC version and leakage values from eFuse and
provides them as matching data to the opp framework.

Change-Id: I10f959edd46668bedf3be4835bb5ec63e089808d
Signed-off-by: Finley Xiao <finley.xiao@rock-chips.com>