Documentation: dt-bindings: Add binding documentation for TI clkctrl clocks

Texas Instruments omap variant SoCs starting with omap4 have a clkctrl
clock controller instance for each interconnect target module. The clkctrl
controls functional and interface clocks for the module.

The clkctrl clocks are currently handled by arch/arm/mach-omap2 hwmod code.
With this binding and a related clock device driver we can start moving the
clkctrl clock handling to live in drivers/clk/ti.

Note that this binding allows keeping the clockdomain related parts out of
drivers/clock. The CLKCTCTRL and DYNAMICDEP registers can be handled by
a separate driver in drivers/soc/ti and genpd. If the clockdomain driver
needs to know it's clocks, we can just set the the clkctrl device
instances to be children of the related clockdomain device.

Each clkctrl clock can have multiple optional gate clocks, and multiple
optional mux clocks. To represent this in device tree, it seems that
it is best done using four clock cells #clock-cells = <2> property.

The reasons for using #clock-cells = <2> are:

1. We need to specify the clkctrl offset from the instance base. Otherwise
   we end up with a large number of device tree nodes that need to be
   patched when new clocks are discovered in a clkctrl clock with minor
   hardware revision changes for example

2. On omap5 CM_L3INIT_USB_HOST_HS_CLKCTRL has ten OPTFCLKEN bits. So we
   need to use a separate cell for optional gate clocks to avoid address
   space conflicts

There is probably no need to list input clocks for each clkctrl clock
instance in the binding. If we want to add them, the standard clocks
binding can be used for that.

For hardware reference, see omap4430 TRM "Table 3-1312. L4PER_CM2 Registers
Mapping Summary" for example. It shows one instance of a clkctrl clock
controller with multiple clkctrl registers.

Cc: Paul Walmsley <paul@pwsan.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Tero Kristo <t-kristo@ti.com>