ARM: OMAP2+: remove DSP platform device

It was added to support DSP Bridge. Since DSP Bridge was removed, and
nothing else is using the platform device, remove it too.

Signed-off-by: Kristina Martšenko <kristina.martsenko@gmail.com>
Cc: Omar Ramirez Luna <omar.ramirez@copitl.com>
Cc: Suman Anna <s-anna@ti.com>
Cc: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>