boards: arm: b_l072z_lrwan1: Enable the USB controller

The STM32L072CZ MCU has a built-in USB device controller which is
supported by Zephyr's USB STM32 driver. The b_l072z_lrwan1 board has a
micro-USB connector that is wired to the MCU via two solder bridges
(SB15 and SB16) that need to be closed to connect the data lines.

Update the board documentation to describe the solder bridges and
enable the USB controller in the device tree.

Signed-off-by: Andreas Sandberg <andreas@sandberg.pp.se>