omap: overo: Add regulator for ads7846

The ads7846 driver now requires a regulator.  This patch adds the
necessary regulator to the overo board file.  Without it, the
following error occurs (and the touchscreen will not function):

ads7846 spi1.0: unable to get regulator: -19

Signed-off-by: Steve Sakoman <steve@sakoman.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>