clk: stm32mp1: move oscillator functions to generic file

Those functions are generic for parsing nodes from device tree
hence could be located in generic source file.

The oscillators description structure is also moved to STM32MP1 clock
driver, as it is no more used in stm32mp1_clkfunc and cannot be in a
generic file.

Change-Id: I93ba74f4eea916440fef9b160d306af1b39f17c6
Signed-off-by: Yann Gautier <yann.gautier@st.com>