ARM: dts: imx6sl: Adding cpu frequency and VDDSOC/PU table.

Device tree for iMX6SL doesn't have an existing cpu frequency table
as well as the VDDSOC/PU.

Signed-off-by: John Tobias <john.tobias.ph@gmail.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>