OMAP3EVM: Add vio regulator supply required for ads7846 TSC driver

Add vio regulator supply, needed for ads7846 touchscreen controller
driver.

Tested on OMAP3 (ES3.1 Si) RevG version of EVM.

Signed-off-by: Vaibhav Hiremath <hvaibhav@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>