soc: arm: atmel: sam4s: Fix Kconfig typo
soc: arm: atmel: sam4s: Fix Kconfig typo

In soc.h reference to CONFIG_SOC_PART_NUMBER_SAM4S18B should have been
CONFIG_SOC_PART_NUMBER_SAM4S8.

Signed-off-by: Kumar Gala <a5e5248af4cd4f0ed8c515f61d40a6e2db46a66e@linaro.org>

nashif/zephyr,Vudentz/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,nashif/zephyr,finikorg/zephyr,galak/zephyr,explora26/zephyr,ldts/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,explora26/zephyr,ldts/zephyr,nashif/zephyr,ldts/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr,GiulianoFranchetto/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,ldts/zephyr,explora26/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,nashif/zephyr,explora26/zephyr,finikorg/zephyr,Vudentz/zephyr,ldts/zephyr,galak/zephyr