Fix KConfig warning

This patch fixes the following Kconfig warning :
"arch/arm/mach-mx25/Kconfig:18:warning: choice value used outside its
choice group

Signed-off-by: Eric Bénard <eric@eukrea.com>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>