drivers: flash: stm32H5 flash driver gives the flash size

Use the LL_GetFlashSize to retrieve the size of the flash, reading the
system flash OTP of the stm32h5 serie. This operation requires
Icache disable/re-enable to access the 0x08FF F80C memory area

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