dt-bindings: clock: Add binding for TI clksel

In order to prepare for fixing lots of devicetree unique_unit_address
warnings for the TI clock nodes, let's add a binding for the TI clksel
clocks. This allows us to move the overlapping reg properties for the
component clocks to be children of the related clksel nodes. And with
that we need the reg property only for the parent clksel node making
the reg property unique like it should be.

We want to set #clock-cells = <2> in case we ever start parsing ranges
of clkcsel instances directly using a clksel driver rather than using the
existing component clock drivers and child nodes.

And before the devicetree files can be updated, we need to update the
TI clock drivers to get the IO address from the parent clksel node.

Cc: Tero Kristo <kristo@kernel.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220203073929.59296-1-tony@atomide.com