mxc_spi refactoring (for mx6q and mx6qsabrelite)

This patch set refactors mxc_spi as described in

     http://lists.denx.de/pipermail/u-boot/2010-March/068791.html
and requested in
     http://lists.denx.de/pipermail/u-boot/2012-January/116023.html

in order to add support for the MX6Q in general and the mx6qsabrelite specifically.

Patch 1 simply moves the conditional parts of mxc_spi.c into the respective CPU-specific imx-regs.h files.

Patch 2 adds general support for SPI to the i.MX6.

Patch 3 adds support to the mx6qsabrelite board

Patch 4 modifies the 'sf' command to allow a default bus and chip-select to be specified by board headers. This allows a bare 'sf' probe command:
U-Boot> sf probe
instead of the more cumbersome usage when a GPIO is tacked onto the chip-select. Otherwise, this command-line would be needed to specify GP3:19 on SabreLite:

      U-Boot> sf probe 0x5300

Patch 5 provides a description of usage and configuration of CONFIG_CMD_SF.

Patch 6 adds default chip-select values for mx6qsabrelite platform.

Patch 7 adds reference macros for use in storing the environment in serial flash to match the use on Freescale's U-Boot release

This patch set has been compiled against the following configurations, but only tested on mx6qsabrelite:

	mx6qsabrelite
	mx51evk
	mx31pdk
	mx35pdk