dts: stm32: Describe default domain clock configuration for usb devices

Describe USB default domain clock on all USB devices.
When available select HSI48.
On some series, a default clock my be set by default at start up.
On those series, in order to be able to compute USB clock at runtime,
clock_control driver needs to be aware of configuration and then this
default config has to be described explicitly too.

Default clocks are not enabled though. It is up to board configuration
to provide correct clock configuration (and we should not enabled by
default clocks that would not be required by board configuration).

Note: This change doesn't consider STM32F1/F3 devices, which
have a specific USB clock configuration.

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