drivers/usb: usb_dc_stm32: Convert to DT_INST

Convert usb_stm32 driver to use of DT_INST macros.

Since driver is compatible with 3 different dt compatibles and
compatible string is included in DT_INST macros, I've kept the
DT_USB_ compatible agnostic macros based on DT_INST ones, which
allowed to remove fixup definitions.
Use of DT_USB symbols is now limited to usb_dc_stm32.

Additionally, compatible "st,stm32-otgfs" is removed from list
of compatibles for usbotg_hs ips.

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>