ARM: dts: imx28-tx28: remove spidev labels and add third instance of spidev

The labels on the spidev nodes are not used and not required, so
remove them. The TX28 supports 3 chipselects on the SPI
interface. Make all those chipselects available to the user.

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