drivers: clock_control: stm32h7: disable PLL1 before configuring it

If the boot loader already switched the system clock to PLL1 we need
to switch back to HSI first and disable PLL1 before we can configure
PLL1. Otherwise the register writes will simply be ignored and we'll
end up with an inconsistent state.

Most of the code has been recycled from `clock_stm32_ll_common.c`.

Signed-off-by: Sascha Silbe <sascha-pgp@se-silbe.de>
Signed-off-by: Sascha Silbe <sascha-pgp@silbe.org>