i2c: add bcm2835 driver

This implements a very basic I2C host driver for the BCM2835 SoC. Missing
features so far are:

* 10-bit addressing.
* DMA.

Reviewed-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>
Signed-off-by: Wolfram Sang <wolfram@the-dreams.de>