spi: imx: GPIO based chip selects should not be required

The driver will fail to load if no gpio chip selects are specified,
this patch changes this so that it no longer fails.

It's possible to use all native chip selects, in which case there is
no reason to have a gpio chip select array.  This is what happens if
the *optional* device tree property "cs-gpios" is omitted.

The spi core already checks for the absence of gpio chip selects in
the master and assigns any slaves the gpio_cs value of -ENOENT.

Also have the driver respect the standard SPI device tree property "num-cs"
to allow setting the number of chip selects without using cs-gpios.

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