serial: 8250: Add OF earlycon support

This allows earlycon to be used without needing to specify the I/O
address on the kernel command line, if linux,stdout-path is specified
in the chosen node.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>