[POWERPC] Open Firmware serial port driver

This can be used for serial ports that are connected to an
OF platform bus but are not autodetected by the lecacy
serial support.
It will automatically take over devices that come from the
legacy serial detection, which usually is only one device.

In some cases, rtas may be set up to use the serial port
in the firmware, which allows easier debugging before probing
the serial ports. In this case, the "used-by-rtas" property
must be set by the firmware. This patch also adds code to the
legacy serial driver to check for this.

Signed-off-by: Arnd Bergmann <arnd.bergmann@de.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>