i2c: core: Add support for 'i2c-bus' subnode

If the 'i2c-bus' device-tree node is present for an I2C adapter then
parse this subnode for I2C slaves.

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