i2c: add SLIMpro I2C device driver on APM X-Gene platform

Add SLIMpro I2C device driver on APM X-Gene platform. This I2C
device driver use the SLIMpro Mailbox driver to tunnel message to
the SLIMpro coprocessor to do the work of accessing I2C components.

Signed-off-by: Feng Kan <fkan@apm.com>
Signed-off-by: Hieu Le <hnle@apm.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>

Documentation/devicetree/bindings/i2c/i2c-xgene-slimpro.txt

@@ -0,0 +1,15 @@
+APM X-Gene SLIMpro Mailbox I2C Driver
+
+An I2C controller accessed over the "SLIMpro" mailbox.
+
+Required properties :
+
+ - compatible : should be "apm,xgene-slimpro-i2c"
+ - mboxes : use the label reference for the mailbox as the first parameter.
+	    The second parameter is the channel number.
+
+Example :
+	i2cslimpro {
+		compatible = "apm,xgene-slimpro-i2c";
+		mboxes = <&mailbox 0>;
+	};