Nokia RX-51: Enable CONFIG_CONSOLE_MUX

After this change both device display and serial console would contain
U-Boto output automatically without any future configuration. This would
allow easier debugging on real device as access to serial console is hard
and also in qemu emulator where it is easier to copy+paste from serial
console as from SDL framebuffer.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Lokesh Vutla <lokeshvutla@ti.com>