ARM: dts: omap4: Add aliases for rproc nodes

Add aliases for the DSP and IPU remoteproc processor
nodes common to all OMAP4 boards. The aliases uses
the stem "rproc".

The aliases can be overridden, if needed, in the
respective board files.

Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>