boards: Move USB pin configuration to device tree for 2 STM32 boards

As a proof of concept, turn USB signals configuration to USB
for nucleo_wb5rg and disco_l475_iot1 boards.

This implicitly remove pull-up on _ID pin, which turn out to
have no side effect.

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