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>

Documentation/devicetree/bindings/i2c/brcm,bcm2835-i2c.txt

@@ -0,0 +1,20 @@
+Broadcom BCM2835 I2C controller
+
+Required properties:
+- compatible : Should be "brcm,bcm2835-i2c".
+- reg: Should contain register location and length.
+- interrupts: Should contain interrupt.
+- clocks : The clock feeding the I2C controller.
+
+Recommended properties:
+- clock-frequency : desired I2C bus clock frequency in Hz.
+
+Example:
+
+i2c@20205000 {
+	compatible = "brcm,bcm2835-i2c";
+	reg = <0x7e205000 0x1000>;
+	interrupts = <2 21>;
+	clocks = <&clk_i2c>;
+	clock-frequency = <100000>;
+};