usb: dwc2: change compatible st,stm32mp1-hsotg to st,stm32mp15-hsotg

usb: dwc2: change compatible st,stm32mp1-hsotg to st,stm32mp15-hsotg

The Linux kernel v5.7-rc1 introduced the compatible "st,stm32mp15-hsotg".

See Linux kernel commit d49850110434 ("dt-bindings: usb: dwc2: add
support for STM32MP15 SoCs USB OTG HS and FS")

This patch updates the supported compatible in DWC2 driver,
removes the add-on done in U-Boot dtsi and keeps the compatible
defined in SOC dtsi arch/arm/dts/stm32mp151.dtsi:

usbotg_hs: usb-otg@49000000 {
	compatible = "st,stm32mp15-hsotg", "snps,dwc2";
	reg = <0x49000000 0x10000>;
...
};

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>