OMAP2, 3: DSS2: DSS: 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 of DSS 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.

DSS 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>