pinctrl: Document "function" + "pins" pinmux binding

Currently the "function" + "groups" combination is the only documented
format for pinmux nodes, although many drivers use "function" + "pins".
Update the generic pinctrl binding to include the "function" + "pins"
combination as well.

Signed-off-by: Andrew Bresticker <abrestic@chromium.org>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Pawel Moll <pawel.moll@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Ian Campbell <ijc+devicetree@hellion.org.uk>
Cc: Kumar Gala <galak@codeaurora.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>