stm32f411re SoC could run at system clock above 84MHz.
This was not taken into account in __setup_flash function which configure
flash latency depending on system clock. This is now corrected.
Assert added to ease error detection.
Change-Id: I49b92256d611ef464171fb1d8812a4c4d3c27ab8
Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>