OMAP2, 3: DSS2: VENC: 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 VENC 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, venc_vdda_dac reading is moved to venc.c.

VENC 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>