[PATCH v4] linux,stdout-path helper

[Sascha Hauer]

The following adds a helper for matching the linux,stdout-path property
in the chosen node and makes use of it in the i.MX serial driver.

changes since v3:

- move code from separate files to drivers/of/base.c and include/linux/of.h

changes since v2:

- move helper to OF core and make it independent of serial devices

changes since v1:

- move it out of the i.MX serial driver and make it generic for serial
  devices.