drivers: usb: make USB VBUS sensing configurable for STM32 devices

Vbus detection is required by self-powered, with external non-USB
supply, devices to have an operational USB peripheral. The voltage
sensing, when enabled, is performed via a designated pin (PA9 is
the most common).

For greater details see section 2.6, ST application note AN4879.

Signed-off-by: Raul Pando <raul.pando@bytegrity.co.uk>