ARM: imx: select syscon for IMX6SL

This is required for building a kernel that enables only
IMX6SL but not IMX6Q, which would get a build error when
syscon is not available.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Shawn Guo <shawn.guo@linaro.org>