pico-pi-imx7d: Unselect CONFIG_CONSOLE_MUX

Unselect CONFIG_CONSOLE_MUX to fix the following
error for the input, output and error interfaces:

U-Boot 2023.10 (Oct 03 2023 - 21:23:18 -0300)
...
In:    No input devices available!
Out:   No output devices available!
Err:   No error devices available!
Net:   eth0: ethernet@30be0000
Hit any key to stop autoboot:  0

Signed-off-by: Fabio Estevam <festevam@denx.de>