powerpc: clean up ssi.txt, add definition for fsl,ssi-asynchronous

Add the definition of the fsl,ssi-asynchronous property to ssi.txt
(documentation of the device tree bindings for the Freescale SSI device).

Also tidy up the layout of ssi.txt.

Signed-off-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>