drivers: clock_control: clock_stm32_ll_h7.c

Reduce code-complexity of stm32_clock_control_init() function, which is
used and exists for both M4/M7 cores.
Replace dublicated code by proper preprocessor guarding.
This change shall reduce code-errors and copy-paste errors since same
functional code is present only once now.
Identify even more common code

Signed-off-by: Alexander Kozhinov <ak.alexander.kozhinov@gmail.com>