tests/drivers/clock_control: stm32u5: Use a clear_clocks overlay

Instead of relying on existing board clock configuration,
use a clear_clocks.overlay file to first reset the clock
configuration to the default .dtsi state, then apply a
new configuration.
This method should be more robust when trying to use on more
boards and has the benefit to provide correct configuration
examples.

This relies on the fact that overlays are applied in the order
they are provided in DTC_OVERLAY_FILE CMake variable.

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