ARM: bcm2835: add I2C controllers to DT

The BCM2835 has 3 identical I2C controllers. Instantiate them all in the
SoC .dtsi file, and enable the relevant two in the Raspberry Pi board
.dts file.

Note that on the Raspberry Pi Model B revision 1, I2C0 is connected to
the general-purpose expansion header, and I2C1 is connected to the camera
connector. Revision 2 of the board swaps these assignments:-(

Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>