drivers: flash: Use dts to configure STM32 OSPI manager clock

Instead of calling __HAL_RCC_OSPIM_CLK_ENABLE() to enable the OSPI
manager clock, we now use a new clock binding in the dts.

In order to avoid confusion between the different clocks, the driver
is modified to select the clock based on their names instead of indexes.

Signed-off-by: Guillaume Gautier <guillaume.gautier-ext@st.com>