ARM: dts: imx28-tx28: add alias for CAN XCVR regulator

This alias is used by U-Boot to enable/disable the regulator depending
on baseboard type.

Signed-off-by: Lothar Waßmann <LW@KARO-electronics.de>
Signed-off-by: Shawn Guo <shawn.guo@freescale.com>