imx6: Add support for Toradex Apalis family of CoMs

imx6: Add support for Toradex Apalis family of CoMs

This patch adds support for the following computer on modules (CoM) from
Toradex[A]:

 Apalis iMX6 Quad 2GB IT - i.MX 6Quad 800MHz, 2GB DDR3, 4GB eMMC
                                             -40° to +85° C Temp
 Apalis iMX6 Quad 1GB    - i.MX 6Quad 1GHz,   1GB DDR3, 4GB eMMC
                                               0° to +70° C Temp
 Apalis iMX6 Dual 1GB IT - i.MX 6Dual 800MHz, 1GB DDR3, 4GB eMMC
                                             -40° to +85° C Temp
 Apalis iMX6 Dual 512MB  - i.MX 6Dual 1GHz, 512MB DDR3, 4GB eMMC
                                               0° to +70° C Temp

I've developed and tested it on Quad 2GB IT v1.1A and Dual 512MB v1.1A
CoMs, using Ixora[B] carrier board v1.0A, but it should hopefuly work on
Eval[C] board as well.

A. https://www.toradex.com/computer-on-modules/apalis-arm-family/nxp-freescale-imx-6
B. https://www.toradex.com/products/carrier-board/ixora-carrier-board
C. https://www.toradex.com/products/carrier-board/apalis-evaluation-board

Flashing/recovery instructions:

 1. Download and compile imx_loader for OpenWrt from
    https://github.com/ynezz/imx_loader

 2. Enter recovery mode as desribed in
    https://developer.toradex.com/knowledge-base/imx-recovery-mode

 3. Connect board via USB to the host computer, check that it's connected
    by lsusb:

    15a2:0054 Freescale Semiconductor, Inc. i.MX 6Dual/6Quad SystemOnChip
              in RecoveryMode

 4. Copy following OpenWrt images to imx_loader directory:
         SPL
         u-boot.img
         u-boot-with-spl.imx
         openwrt-imx6-apalis-recovery.scr
         openwrt-imx6-apalis-squashfs.combined.bin

 5. Run imx_usb in imx_loader directory

Signed-off-by: Petr Štetiar <ynezz@true.cz>