OMAP2, 3: DSS2: DSI: create platform_driver, move init, exit to driver

Hwmod adaptation design requires each of the DSS HW IP to be a platform driver.
So a platform_driver for DSI is created and init exit methods are moved from core.c
to its driver probe,remove. pdev member has to be maintained by its own drivers.

Also, vdds_dsi regulator handling is copied to dsi.c, since vdds_dsi regulator is
needed by dpi_init() too. Board files are updated accordingly to add 2 instances of
vdds_dsi regulator.

DSI platform driver is registered from inside omap_dss_probe, in the order desired.

Signed-off-by: Senthilvadivu Guruswamy <svadivu@ti.com>
Signed-off-by: Sumit Semwal <sumit.semwal@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>