dt-bindings: i2c: Add support for 'i2c-bus' subnode

dt-bindings: i2c: Add support for 'i2c-bus' subnode

The I2C driver core for boards using device-tree assumes any subnode of
an I2C adapter in the device-tree blob is an I2C slave device. Although
this makes complete sense, some I2C adapters may have subnodes which
are not I2C slaves but subnodes presenting other features. For example
some Tegra devices have an I2C interface which may share its pins with
other devices. In order to share these pins using the pinctrl framework,
it is necessary to add subnodes to the I2C device node that represent
these pins.

To allow I2C adapters to have non-I2C specific subnodes in device-tree
that are not parsed by the I2C driver core, add support for an optional
'i2c-bus' subnode where I2C slaves can be placed. If the 'i2c-bus'
subnode is present then all I2C slaves must be placed under this
subnode.

Signed-off-by: Jon Hunter <jonathanh@nvidia.com>
Acked-by: Thierry Reding <treding@nvidia.com>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Wolfram Sang <wsa@the-dreams.de>
Signed-off-by: Thierry Reding <treding@nvidia.com>