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

+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>;
+	};

drivers/i2c/busses/Kconfig

@@ -1110,6 +1110,15 @@ config I2C_CROS_EC_TUNNEL
 	  connected there. This will work whatever the interface used to
 	  talk to the EC (SPI, I2C or LPC).
 
+config I2C_XGENE_SLIMPRO
+	tristate "APM X-Gene SoC I2C SLIMpro devices support"
+	depends on ARCH_XGENE && MAILBOX
+	help
+	  Enable I2C bus access using the APM X-Gene SoC SLIMpro
+	  co-processor. The I2C device access the I2C bus via the X-Gene
+	  to SLIMpro (On chip coprocessor) mailbox mechanism.
+	  If unsure, say N.
+
 config SCx200_ACB
 	tristate "Geode ACCESS.bus support"
 	depends on X86_32 && PCI

drivers/i2c/busses/Makefile

@@ -110,6 +110,7 @@ obj-$(CONFIG_I2C_ELEKTOR)	+= i2c-elektor.o
 obj-$(CONFIG_I2C_OPAL)		+= i2c-opal.o
 obj-$(CONFIG_I2C_PCA_ISA)	+= i2c-pca-isa.o
 obj-$(CONFIG_I2C_SIBYTE)	+= i2c-sibyte.o
+obj-$(CONFIG_I2C_XGENE_SLIMPRO) += i2c-xgene-slimpro.o
 obj-$(CONFIG_SCx200_ACB)	+= scx200_acb.o
 
 ccflags-$(CONFIG_I2C_DEBUG_BUS) := -DDEBUG