stm32mp1: migrate USBOTG device to driver model

Use the DWC2 device driver with DM_USB_GADGET support and
cleanup the USB support in STM32MP1 board.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
diff --git a/configs/stm32mp15_basic_defconfig b/configs/stm32mp15_basic_defconfig
index b6c1972..fd164fa 100644
--- a/configs/stm32mp15_basic_defconfig
+++ b/configs/stm32mp15_basic_defconfig
@@ -69,6 +69,7 @@
 CONFIG_STM32_SERIAL=y
 CONFIG_USB=y
 CONFIG_DM_USB=y
+CONFIG_DM_USB_GADGET=y
 CONFIG_USB_EHCI_HCD=y
 CONFIG_USB_EHCI_GENERIC=y
 CONFIG_USB_GADGET=y