ARM: OMAP3: cm-t35: Add regulator for ads7846 touchscreen

ads7846 driver fails to find the regulator supply and
as a result the touchscreen is not working.
Fix this by adding a regulator supply for the ads7846 driver.

Signed-off-by: Igor Grinberg <grinberg@compulab.co.il>
[tony@atomide.com: updated subject]
Signed-off-by: Tony Lindgren <tony@atomide.com>