spi: spi-fsl-dspi: Remove spi-bitbang

DSPI module need cs change information in
a spi transfer. According to cs change, DSPI
will give last data the right flag. Bitbang
provide cs change behind the last data in
a transfer. So DSPI can not deal the last
data in every transfer properly, so remove
the bitbang in the driver.

Signed-off-by: Bhuvanchandra DV <bhuvanchandra.dv@toradex.com>
Signed-off-by: Mark Brown <broonie@kernel.org>