i2c: bcm2835: Linking platform nodes to adapter nodes

In order to find I2C devices in the device tree, the platform nodes
have to be known by the I2C core. This requires setting the
dev.of_node parameter of the adapter.

Signed-off-by: Florian Meier <florian.meier@koalo.de>
Tested-by: Stephen Warren <swarren@wwwdotorg.org>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>