configs: am65x_evm_a53: disable CONSOLE_MUX

We do not have a need to share a single console with the evaluation
board and disabling this option reduces the complexity of configuring
the consoles. Disable CONSOLE_MUX

Tested-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Bryan Brattlof <bb@ti.com>
Reviewed-by: Nishanth Menon <nm@ti.com>