drivers: clock_control: stm32 common: implement get_status api

The get_status api allows to query whether clock is turned on or off.
Like the get_rate call, the status of gated clocks (for specific
peripherals), as well as domain clock sources(HSE, PCLK, PLL_P,..)
can be checked this way.

Signed-off-by: Thomas Stranger <thomas.stranger@outlook.com>