drivers: flash: stm32h7x: use barriers to ensure the writes are flushed
drivers: flash: stm32h7x: use barriers to ensure the writes are flushed

The STM32H7 flash driver read-back a register after writing it to ensure
it is flushed. This is very fragile and might break if a new compiler
version slightly reorder the instructions. Instead use a __DSB()
barrier like done on other STM32 SoC, which ensures that the registers
and the data writes are flushed at the compiler level, but also at the
Cortex-M7 level.

Signed-off-by: Aurelien Jarno <bceb368e7f2cb351af47298f32034f0587bbe4a6@aurel32.net>

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