pinctrl: dt-bindings: Fix some errors in the lgm and pinmux schema

This fixes some problems that caused build errors in the
lgm-io schema file:

- No "bindings" infix in the schema id
- Move the allOf inclusion for pinconf and pinmux nodes into
  the patternProperties for the -pins node
- We want "groups" not "group" to be compulsory for a pinmux
  node blended with a pin config node.
- Fix the generic pinmux-schema to list "groups" rather than
  "group" for a pinmux node, this might have led to some confusion.

This is a first user of the generic schema so a bit of a bumpy
road.

Cc: Rob Herring <robh+dt@kernel.org>
Cc: Rahul Tanwar <rahul.tanwar@linux.intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>