fdt_support: Add multi-serial support for stdout fixup

Currently fdt_fixup_stdout() is using hard-coded CONFIG_CONS_INDEX
constant. With multi-serial support, the CONS_INDEX may no longer
represent actual console, so we should try to extract port number
from the current stdio device name instead of always hard-coding the
constant value.

Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Acked-by: Gerald Van Baren <vanbaren@cideas.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>