Prepare for pinctrl-single yaml binding and unify pin group node names.

Let's standardize on pin group node naming ending in -pins. As we don't
necessarily have a SoC specific compatible property for pinctrl-single.
I'd rather not add a pattern match for pins somewhere in the name for all
the users.

Trying to add matches for pins-default will be futile as on the earlier
SoCs we've already seen names like pins-sleep, pins-idle, pins-off and so
on that would need to be matched.

And as the node is a pin group, let's prefer to use naming -pins rather
than -pin as more pins may need to be added to the pin group later on.

Signed-off-by: Tony Lindgren <tony@atomide.com>
[vigneshr@ti.com: Rebase onto latest ti/next and extend to new nodes]
Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>