drivers: flash: stm32 xspi flash driver skip init when executing in place

The flash_stm32_xspi driver should not initialize the xspi,
if this one is being use to execute in Place : the init is skipped.
This mode is identified with the CONFIG_STM32_APP_IN_EXT_FLASH.
Checking the memory mapped mode bit is possible when the xspi
peripheral clock is not off (stm32h5 has no clock_control_get_status API)

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