clk: stm32: retrieve external oscillator frequency from DT

All current STM32F4 supported boards uses a 8MHz external oscillator.
All current STM32F7 supported boards uses a 25MHz external oscillator.

In order to introduce the new stm32f429-evaluation board which uses a
25MHz external oscillator without creating a dedicated struct
stm32_clk_info for this board, retrieve the external oscillator
frequency from DT and set pll_m accordingly to obtain 1MHz for the VCO.

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>