ARM: OMAP2+: Prepare dm81xx hwmod code for adding minimal dm814x support

Let's change the defines so we can share the hwmod code better between
dm816x and dm814x, and let's add the dm814x specific defines. And let's
rename the shared ones to start with dm81xx. No functional changes.

Cc: Matthijs van Duin <matthijsvanduin@gmail.com>
Cc: Paul Walmsley <paul@pwsan.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>