soc: stm32f407xg: rename SOC config flag

Rename SOC_STM32F407XX to SOC_STM32F407XG 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: I0afa16e86b7c99b9e685004f96beeb888f9e7568
Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>