ARM: dts: cfa10049: Change the SPI3 bus to spi-gpio

The DAC found on the last chip select requires a word length of 12 bits,
which is not supported by the SSP controller of the iMX28. Use
bitbanging for that bus to support such a length.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>