uboot-imx

dm: serial: Separate out the core serial-device finding code

This function is quite long. Move the core code into a separate function
in preparation for adding livetree support.

Signed-off-by: Simon Glass <sjg@chromium.org>
Tested-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Tested-on: Beaver, Jetson-TK1