clk: imx7ulp: Correct DDR clock mux options

In the latest reference manual Rev.0,06/2019, the DDR clock mux
is extended to 2 bits, and the clock options are also changed,
correct them accordingly.

Fixes: b1260067

 ("clk: imx: add imx7ulp clk driver")
Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>