dm: serial: Handle "stdout-path" with ":options" correctly

dm: serial: Handle "stdout-path" with ":options" correctly

With commit 87361ea25116 ("fdt: Sync up to the latest libfdt"),
SiFive Unleashed board does not boot any more. This was due to
the U-Boot local changes commit 77d7fff8cec2 ("fdt: Fix handling
of paths with options in them") to libfdt/fdt_ro.c was dropped
during the libfdt upgrade.

>From the history [1] it was mentioned that the U-Boot changes
commit 77d7fff8cec2 ("fdt: Fix handling of paths with options in
them") was rejected by libfdt upstream, hence we need find another
way to fix the things.

This commit uses another method, by updating serial_check_stdout()
directly to handle the situation of "stdout-path" with ":options".
A simpler way is to change the logic in fdtdec_get_chosen_node()
to do similar thing, but I feel that not every property in chosen
node may have the option in them, hence it would make more sense
to do the special handling in serial_check_stdout() directly.

[1]: http://patchwork.ozlabs.org/patch/462756/

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>