drivers: flash: stm32H7 flash driver enable clock when defined

Not all the stm32H7 have a clock enable for their flash;
only the dual-core.
For the st,stm32h7-flash-controller with "clocks" property, the driver
will enable the flash clock bit in the corresponding RCC register.

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