sunxi: Do not add a stdout-path alias to dts on boards without a serial port

Do not add a bogus (pointing to a non existing serial port) stdout-path
alias to dts on boards without a serial port.

Note that we still define CONS_INDEX as this is used by the SPL where we do
not use DM_SERIAL and thus CONFIG_REQUIRE_SERIAL_CONSOLE is not honored.

We are getting away with this because the sun5i die actually has
an uart0, which in the A13 package is not routed to the outside,
so we are simply sending SPL bootup messages to the tx pin at the
edge of the die, and they go no further from there...

And sofar we only have one A13 board which does not have a serial
port, all others do have a serial port. This kinda makes sense since
the A13 is a much lower pincount package compared to all the other
sunxi SoCs.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Ian Campbell <ijc@hellion.org.uk>