drivers: clock_control: stm32h7: disable PLLs before configuration

Disable every PLL before configuration. That allows
an application to reconfigure PLLs after a bootloader
configuration.

Don't disable the PLL clock if it is used by (Q|O)SPI
when executing from external memory. That will lead
to a stall.

Note: when (Q|O)SPI runs from PLL, the bootloader
dictates the clock configuration. There is no clock
reconfiguration support for memory map mode in
(Q|O)SPI drivers.

Signed-off-by: Georgij Cernysiov <geo.cgv@gmail.com>