spi/mpc8xxx: rename spi_mpc8xxx.c to spi_fsl_spi.c

This will pave the way to refactor out the common code which can be used
by the eSPI controller driver, and rename the SPI controller dirver to the
file spi_fsl_spi.c.

Signed-off-by: Mingkai Hu <Mingkai.hu@freescale.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>