drivers/clock_control: stm32: Add CLOCK_CONTROL_STM32_HAS_DTS

Add new symbol CLOCK_CONTROL_STM32_HAS_DTS to exclude definition
of other CLOCK_CONTROL_STM32_* symbols when dts based configuration
is in use.
CLOCK_CONTROL_STM32_HAS_DTS is defined based on availability of
"clocks" property in rcc node.

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