clk: imx7ulp: Correct system clock source option #7

In the latest reference manual Rev.0,06/2019, the SCS's option #7
is no longer from upll, it is reserved, update clock driver 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>