soc: stm32f429zi: rename SOC config flag

Rename SOC_STM32F429XX to SOC_STM32F429XI to keep flash
size information.
Aim is to be able to distinguish flash size variants of
the SoC when needed (for instance in dts/arm/st/mem.h file)

Change-Id: Id188b7703d2bce0a3ded09132ff0f205efa9c143
Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>