omap: rx51: Add vdda_dac supply for tvout

Commmit 60d24eed

 "Added video data to support tvout on rx51" broke the DSS
on RX51/N900 since it added DSS VENC support but a patch adding needed
supply is missing from tree and no framebuffers are initialized.

This patch is basically cleaned up version of original one:
http://marc.info/?l=linux-omap&m=129070041402418&w=2

Signed-off-by: Jarkko Nikula <jhnikula@gmail.com>
Cc: Srikar <ext-srikar.1.bhavanarayana@nokia.com>
Cc: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Tomi Valkeinen <tomi.valkeinen@nokia.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>