spi/fsl_spi: fix CPM spi driver

This patch fixes the freescale spi driver for CPM. Without this
patch SPI on CPM failed because cpm_muram_alloc_fixed tries to
allocate muram in an preserved area. The error reported was:

mpc8xxx_spi f0011a80.spi: can't allocate spi parameter ram
mpc8xxx_spi: probe of f0011a80.spi failed with error -12

Now the driver uses of_iomap to get access to this area
similar to i2c driver driver in the i2c-cpm.c which has a
similar device tree node. This is tested on a MPC8247 with CPM2.

Signed-off-by: Holger Brunck <holger.brunck@keymile.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>