dt: document the of_serial bindings

The of_serial bindings can be used to register a number of serial
devices.  Document this binding with all of the others.

v3: remove device-type and clarify used-by-rtas

Signed-off-by: Jamie Iles <jamie@jamieiles.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>