drivers/flash: stm32f3/stm32f1: define flash registers as volatile

On STM32F3 devices, a trick was required for normal use of registers.
This was actually an issue in flash registers defintions which should
be defined as volatile.
Fix this and additionaly, fix definition for STM32F1 which was also
lacking the volatile instruction.

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>