arm: dts: imx7ulp-evk: remove mux value from pad configuration

The mux mode is embedded in the PAD definition and therefore there is
no need to repeat it in the PAD configuration value (more over since
this information will be masked out when the configuration value is
applied).

Signed-off-by: Jorge Ramirez-Ortiz <jorge@foundries.io>
Reviewed-by: Fabio Estevam <festevam@gmail.com>