ARM: imx: add clock support for imx6sl

Add clock support for i.MX6 SoloLite.  It uses the dtc marco support to
define all clock IDs in imx6sl-clock.h, which will be included by both
clock driver and device tree sources, so that the data will stay sync
all the time between kernel and DT.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>