spi/imx: get rid of more ifs depending on the used cpu

Nearly everything that is needed is provided by the version of the SPI IP.
Now the only checks left using cpu_is_... are clk divider tuning on mx21/mx27
and autodetection (which will die soon).

Acked-by: Jason Wang <jason77.wang@gmail.com>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>