Rename and export dsi_vc_enable_hs() so that the display drivers can
control the mode of the DSI link.
Signed-off-by: NTomi Valkeinen <tomi.valkeinen@nokia.com>

Add helper functions for most common DCS read and write operations.
Signed-off-by: NTomi Valkeinen <tomi.valkeinen@nokia.com>

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: NTomi Valkeinen <tomi.valkeinen@nokia.com>