dts: bindings: clocks stm32: Clock selection rewording and clarification

Change the wording about "alternate/optional" clock to "domain" clocks,
to better describe the feature and minimize confusion.

Additionally, provide more  description about domain clocks and dual
domain functionality implemented in peripherals.

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