spi: imx: Don't require platform data chipselect array

If the array is not present, assume all chip selects are native.  This
is the standard behavior for SPI masters configured via the device
tree and the behavior of this driver as well when it is configured via
device tree.

This reduces platform data vs DT differences and allows most of the
platform data based boards to remove their chip select arrays.

CC: Shawn Guo <shawnguo@kernel.org>
CC: Sascha Hauer <kernel@pengutronix.de>
CC: Fabio Estevam <fabio.estevam@nxp.com>
CC: Mark Brown <broonie@kernel.org>
Signed-off-by: Trent Piepho <tpiepho@impinj.com>
Signed-off-by: Mark Brown <broonie@kernel.org>