OMAP: DSS2: Taal: Implement ULPS functionality

ULPS is a low power state where the DSI lanes are kept at ground. This
patch implements ULPS by having a DSI bus inactivity timer which
triggers the entry to ULPS. ULPS exit will happen automatically when the
driver needs to do something on the DSI lanes.

The ulps_timeout is configurable from board file or via sysfs.
Additionally another sysfs file, "ulps", can be used to check the
current ULPS state, or to manually enter or exit ULPS.

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