soc: stm32f429zi: finish SOC renaming

Commit 87893ddf7ad4 ("soc: stm32f429zi: rename SOC config flag") renamed
SOC_STM32F429XX to SOC_STM32F429XI but the text of the option should be
changed as well to reflect this restriction in scope.

Change-Id: I2627b59f805e73d6c8a3534e0feec71a4269c9ab
Signed-off-by: Florian Vaussard <florian.vaussard@heig-vd.ch>
1 file changed