soc: stm32l476rg: rename SOC config flag

Rename SOC_STM32L476XX to SOC_STM32L476XG 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: I834bb5b83c24c39e90c0492a2b22a7c7802de361
Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>