stm32mp1: add stusb1600 support for DK1 and DK2 board

The DK1 and DK2 boards use the USB Type-C controller STUSB1600.
This patch updates:
- the device tree to add the I2C node in the DT
- the board stm32mp1 to probe this I2C device and use this controller
  to check cable detection.
- the DWC2 driver to support a new dt property
  "u-boot,force-b-session-valid" which forces B session and
  device mode; it is a workaround because the VBUS sensing and
  ID detection isn't available with stusb1600.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Reviewed-by: Lukasz Majewski <lukma@denx.de>
diff --git a/arch/arm/dts/stm32mp157-pinctrl.dtsi b/arch/arm/dts/stm32mp157-pinctrl.dtsi
index c069875..0aae69b 100644
--- a/arch/arm/dts/stm32mp157-pinctrl.dtsi
+++ b/arch/arm/dts/stm32mp157-pinctrl.dtsi
@@ -375,6 +375,13 @@
 				};
 			};
 
+			stusb1600_pins_a: stusb1600-0 {
+				pins {
+					pinmux = <STM32_PINMUX('I', 11, ANALOG)>;
+					bias-pull-up;
+				};
+			};
+
 			uart4_pins_a: uart4-0 {
 				pins1 {
 					pinmux = <STM32_PINMUX('G', 11, AF6)>; /* UART4_TX */