ARM: dts: stm32: enable usb-role-switch on USB OTG on stm32mp15xx-dkx

Now that USB OTG driver supports usb role switch by overriding PHY input
signals (A-Valid, B-Valid and Vbus-Valid), enable it on stm32mp15xx-dkx.
dr_mode needn't to be forced to Peripheral anymore, it is set to OTG in
SoC device tree.
USB role (USB_ROLE_NONE, USB_ROLE_DEVICE, USB_ROLE_HOST) will be provided
by STUSB1600 Type-C controller driver.

This patch depends on "Add STUSB160x Type-C port controller support"
series, which is under review.

Signed-off-by: Amelie Delaunay <amelie.delaunay@st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>