drivers: clock: stm32H5 clock control of the PLL in MemoryMapped mode

Do not disable the PLL clock if it sources the XSPI and if the external
flash is executing in Place. After mcuboot reset, the code is executed
on the external flash, through the xspi.
The CONFIG_STM32_APP_IN_EXT_FLASH is set and will avoid re-config
of the PLL which is sourcing the XSPI peripheral. When eXecuting in Place
on this external NOR, it must not disable its own clock source (PLL).

Signed-off-by: Francois Ramu <francois.ramu@st.com>