i2c: add i2c driver for stm32

Add i2c driver which can be used on both STM32F7 and STM32H7.
This I2C block supports the following features:
 _ Slave and master modes
 _ Multimaster capability
 _ Standard-mode (up to 100 kHz)
 _ Fast-mode (up to 400 kHz)
 _ Fast-mode Plus (up to 1 MHz)
 _ 7-bit and 10-bit addressing mode
 _ Multiple 7-bit slave addresses (2 addresses, 1 with configurable mask)
 _ All 7-bit addresses acknowledge mode
 _ General call
 _ Programmable setup and hold times
 _ Easy to use event management
 _ Optional clock stretching
 _ Software reset

Signed-off-by: Christophe Kerello <christophe.kerello@st.com>
Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

doc/device-tree-bindings/i2c/i2c-stm32.txt

@@ -0,0 +1,30 @@
+* I2C controller embedded in STMicroelectronis STM32 platforms
+
+Required properties :
+- compatible : Must be "st,stm32f7-i2c"
+- reg : Offset and length of the register set for the device
+- resets: Must contain the phandle to the reset controller
+- clocks: Must contain the input clock of the I2C instance
+- A pinctrl state named "default" must be defined to set pins in mode of
+  operation for I2C transfer
+- #address-cells = <1>;
+- #size-cells = <0>;
+
+Optional properties :
+- clock-frequency : Desired I2C bus clock frequency in Hz. If not specified,
+  the default 100 kHz frequency will be used. As only Normal, Fast and Fast+
+  modes are implemented, possible values are 100000, 400000 and 1000000.
+
+Example :
+
+	i2c1: i2c@40005400 {
+		compatible = "st,stm32f7-i2c";
+		reg = <0x40005400 0x400>;
+		resets = <&rcc 181>;
+		clocks = <&clk_pclk1>;
+		pinctrl-names = "default";
+		pinctrl-0 = <&pinctrl_i2c1>;
+		clock-frequency = <400000>;
+		#address-cells = <1>;
+		#size-cells = <0>;
+	};