omap3sdp: Fix regulator mapping for ads7846 TS controller

On the OMAP3430SDP board, the ads7846 touchscreen controller
is powered by VAUX3 regulator (supplying 2.8v).
Fix this mapping in the board file, and hence prevent
the ads7846 driver init to fail with the below error..

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

Signed-off-by: Rajendra Nayak <rnayak@ti.com>
Reviewed-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>