OMAP: DSS2: Add FEAT_VENC_REQUIRES_TV_DAC_CLK

OMAP3430 requires an 96MHz clock to VENC's DAC, but no other OMAP needs
it.

Add a new feature, FEAT_VENC_REQUIRES_TV_DAC_CLK, which tells if the
clock is needed on this platform, and use that feature in venc.c to
decide if the clock needs enabling.

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