dts/arm/st: stm32f3: Provide erase and write block size

On currently supported stm32f3 chips,
erase-block-size = 2048
write-block-size = 2
Set these property in stm32f3 series root dtsi file.

Fixes #9686

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