spi/imx: use mx21 to name SPI_IMX_VER_0_0 function and macro

SPI_IMX_VER_0_0 covers i.mx21 and i.mx27.  It makes more sense to
use mx21 rather than mx27 to name SPI_IMX_VER_0_0 function and
macro, since i.mx21 comes out ealier than i.mx27.

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>