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

OMAP2, 3: DSS2: RFBI: 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 RFBI 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.

RFBI 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>