Documentation: dt: add bindings for ti-cpufreq

Add the device tree bindings document for the TI CPUFreq/OPP driver
on AM33xx, AM43xx, DRA7xx, and AM57xx SoCs. The operating-points-v2
binding allows us to provide an opp-supported-hw property for each OPP
to define when it is available. This driver is responsible for reading
and parsing registers to determine which OPPs can be selectively enabled
based on the specific SoC in use by matching against the opp-supported-hw
data.

Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Dave Gerlach <d-gerlach@ti.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>