dt-bindings: w1: document generic onewire bindings

This patch adds a generic w1 bindings document that describes how w1
slave deviceses are grouped under master nodes. It also augments the
existing w1-gpio.txt document a bit.

Signed-off-by: Daniel Mack <daniel@zonque.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>