clk: imx: imx6sx: update cko mux options

According to latest reference manual (Rev.2, 9/2017),
previous CKO1/2's mux options are incorrect, update
them.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>