spi/imx: do not use spi_imx2_3 to name SPI_IMX_VER_2_3 function and macro

It's confusing to use spi_imx2_3 to name SPI_IMX_VER_2_3 function
and macro, as it easily make people think of imx2 and imx3.  It's
better to use specific soc name just like what other SPI_IMX_VER
do.  For SPI_IMX_VER_2_3 case, it will be mx51.  To distinguish it
from CSPI on mx51, mx51_ecspi might be a good choice.

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>