drivers/clock_control: stm32: Move CLOCK_STM32_HSE_CLOCK definition

Symbol CLOCK_STM32_HSE_CLOCK will remain in use in context of stm32
clock_control configuration using device tree, cf commit
a7989f64.

In preparation for next change, separate it from the others symbols
definition. Also make it non dependent from other Kconfig symbols.

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