soc: stm32f407xg: finish SOC renaming

Commit 599149dfb831 ("soc: stm32f407xg: rename SOC config flag") renamed
SOC_STM32F407XX to SOC_STM32F407XG but the text of the option should be
changed as well to reflect this restriction in scope.

Change-Id: Id03529452f5ec7d7ffee214b152c4aa555e1208a
Signed-off-by: Florian Vaussard <florian.vaussard@heig-vd.ch>
1 file changed