ARM: dts: imx28-tx28: add spi-gpio as alternative for spi-mxs

The spi-mxs driver does not allow full duplex SPI transfers. The
spi-gpio driver may be used as an alternative if this is required.

Make the choice between those drivers easier for the end user by
providing settings for both drivers.

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