omap: add dsp platform device

Otherwise tidspbridge cannot work.

It looks like this was dropped in the conversion to staging. I took the
liberty of doing some cleaning up.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>