usb: dwc3: add ST dwc3 glue layer to manage dwc3 HC

This patch adds the ST glue logic to manage the DWC3 HC
on STiH407 SoC family. It manages the powerdown signal,
and configures the internal glue logic and syscfg registers.

[ balbi@ti.com : actually switch over to of_platform_depopulate() ]
Signed-off-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: Peter Griffin <peter.griffin@linaro.org>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Felipe Balbi <balbi@ti.com>

drivers/usb/dwc3/Kconfig

@@ -80,6 +80,15 @@ config USB_DWC3_KEYSTONE
 	  Support of USB2/3 functionality in TI Keystone2 platforms.
 	  Say 'Y' or 'M' here if you have one such device
 
+config USB_DWC3_ST
+	tristate "STMicroelectronics Platforms"
+	depends on ARCH_STI && OF
+	default USB_DWC3
+	help
+	  STMicroelectronics SoCs with one DesignWare Core USB3 IP
+	  inside (i.e. STiH407).
+	  Say 'Y' or 'M' if you have one such device.
+
 comment "Debugging features"
 
 config USB_DWC3_DEBUG

drivers/usb/dwc3/Makefile

@@ -36,3 +36,4 @@ obj-$(CONFIG_USB_DWC3_OMAP)		+= dwc3-omap.o
 obj-$(CONFIG_USB_DWC3_EXYNOS)		+= dwc3-exynos.o
 obj-$(CONFIG_USB_DWC3_PCI)		+= dwc3-pci.o
 obj-$(CONFIG_USB_DWC3_KEYSTONE)		+= dwc3-keystone.o
+obj-$(CONFIG_USB_DWC3_ST)		+= dwc3-st.o