tests: clock_control: stm32h7_devices: Test perck configuration

When it needs to access perck clock speed, clock_control driver is using
LL API to read RCC registers and compute frequency.
We're using the exact same method to test the frequency and as a result
we were not able to detect that there was an issue when configuring this
clock.

Add a specific case to this test in order to verify perck domain clock if
perck is used in SPI clk configuration.

We're now able to detect issues (and test is failing).

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