of: support passing console options with stdout-path

Support specifying console options (like with console=ttyXN,<options>)
by appending them to the stdout-path property after a separating ':'.

Example:
        stdout-path = "uart0:115200";

Signed-off-by: Leif Lindholm <leif.lindholm@linaro.org>
[grant.likely: minor rework to shorten the diffstat]
Signed-off-by: Grant Likely <grant.likely@linaro.org>