OMAP: DSS2: Display Subsystem Driver core

The core files of DSS2. DSS2 commits are split a bit artificially to
make the individual commits smaller, and DSS2 doesn't compile properly
without the rest of the core commits. This shouldn't be a problem, as no
configuration uses DSS2 yet.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@nokia.com>