mx28evk: add SPI support

This patch adds SPI support for the MX28EVK. Support for
an optionally installed SPI flash is also added. An example
configuration for redundant envrionment from SPI flash is also
added but disabled by default.

This patch has been tested on a MX28EVK Rev. D with an installed
SST25VF032B 32Mbit SPI flash.

Signed-off-by: Matthias Fuchs <matthias.fuchs@esd.eu>
Acked-by: Fabio Estevam <fabio.estevam@freescale.com>