dt-bindings: pinctrl: rt2880: properly redo bindings

dt-bindings: pinctrl: rt2880: properly redo bindings

When this bindings where applied there weren't already
reviewed and some old hacks was being used to properly
pass the schemas checks. This commits fix them up:
 - Instead of using 'if-then' clause use '-pins$'.
 - 'groups' and 'function' are included inside a new
   '^(.*-)?pinmux$' node.
 - compatible string is not an 'enum' but a 'const'.
 - 'pinctrl-0' and 'pinctrl-names' removed since they are
    used in consumer nodes.

Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20201213161721.6514-2-sergio.paracuellos@gmail.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>