arch: stm32l432: add support for USB controller

The STM32L432 SoC has a standard non-OTG USB controller. Add an entry
for it in stm32l432.dtsi and add the corresponding DTS fixup entries.
The controller is kept disabled and should be enabled at the board
level.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>