After following documentation found at : https://wiki.st.com/stm32mpu/wiki/OTG_device_tree_configuration
we can not bring FS OTG up using mainline Linux (5.10). We updated the compatible string to st,stm32mp15-fsotg, but without effect.

When booting linux we get this error :
[    9.724039] dwc2 49000000.usb-otg: dwc2_core_reset: HANG! Soft Reset timeout GRSTCTL_CSFTRST

Segments from dts file :

vbus_otg: regulator_otg {
                compatible = "regulator-fixed";
                regulator-name = "vdd-otg";
                regulator-min-microvolt = <5000000>;
                regulator-max-microvolt = <5000000>;
                enable-active-high;
                gpio = <&gpiog 0 0>;
        };
.........................
&usbphyc {
        status = "okay";
};

&usbh_ehci {
        phys = <&usbphyc_port0>, <&usbphyc_port1 1>;
        phy-names = "usb1-phy", "usb2-phy";
        status = "okay";
};

&usbphyc_port0 {
        phy-supply = <&vdd_usb>;
        vbus-supply = <&usb1vbus>;
        vdda1v1-supply = <&reg11>;
        vdda1v8-supply = <&reg18>;
};

&usbphyc_port1 {
        phy-supply = <&vdd_usb>;
        vbus-supply = <&usb2vbus>;
        vdda1v1-supply = <&reg11>;
        vdda1v8-supply = <&reg18>;
};

&usbotg_hs {
        compatible = "st,stm32mp15-fsotg", "snps,dwc2";
        pinctrl-names = "default";
        pinctrl-0 = <&usbotg_hs_pins_a &usbotg_fs_dp_dm_pins_a>;
        vbus-supply = <&vbus_otg>;
        dr_mode = "peripheral";
        vusb_d = <&reg11>;
        vusb_a = <&reg18>;
        status = "okay";
};
................................
We use a custom board having 3 USB ports, two high speed usb ports and one FS OTG port.