tests/drivers/clock_control: stm32u5: Rework to explicitly test HCLK

Instead of testing SysClockFreq setting, we should instead check HCLK
setting which is the real zephyr CONFIG_SYS_CLOCK_HW_CYCLES_PER_SEC
counterpart (core clock freq) and takes AHB prescaler setting into
account.

Additionally, update one test configuration to explicitly verify AHB
prescaler is correctly taken into account by clock driver.

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