dm: serial: Deal with stdout-path with an alias

Sometimes stdout-path contains a UART alias along with speed, etc. For
example:

	stdout-path = "serial0:115200n8";

Add support for decoding this.

Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>