spi/imx: use soc name in spi device type naming scheme

Software defined version number is not stable enough to be used
in device type naming scheme.  The patch changes it to use implicit
soc name for spi device type definition.  In this way, we can easily
align the naming scheme with device tree binding, which comes later.

It removes fifosize from spi_imx_data and adds devtype there, so that
fifosize can be set in an inline function according to devtype.
Also, cpu_is_mx can be replaced by inline functions checking devtype.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Cc: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Cc: Sascha Hauer <s.hauer@pengutronix.de>
Cc: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>