Documentation: DT: omap_serial: document missing properties and add an example

The omap_serial.txt binding documentation lacks a number of properties
that are used in DTS files for platforms incorporating this peripheral.
Fix this by documenting the missing required and optional fields and
add an example.

Signed-off-by: Matt Porter <mporter@konsulko.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>