tests/drivers/clock_control: stm32_common: Test HCLCK instead of SYSCLK

Rework test_*_freq to test HCLK freq instead of SYSCLK one, as it is not
correct to compare CONFIG_SYS_CLOCK_HW_CYCLES_PER_SEC with SYSCLK.

Additionally, add a test to verify use of AHB prescaler.

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