drivers: clock: get_pllsrc_frequency for stm32f2/f4/f7

The function get_pllsrc_frequency is added
with possible source HSI or HSE.
With STM32_PLL_P_ENABLED or STM32_PLL_Q_ENABLED,
this function is used by the clock control driver.

Signed-off-by: Francois Ramu <francois.ramu@st.com>