drivers: udc_rpi_pico: support VBUS state change detection

Support VBUS state change detection and enable/disable DP pull-up
according to VBUS state when pinctrl property is provided.

It brings the similar functionality introduced in commit 4c0317fa
("drivers: usb_dc_rpi_pico: Implemented vbus detection handling")
for the legacy device controller driver.

Signed-off-by: Johann Fischer <johann.fischer@nordicsemi.no>