imx: add support for Toradex Colibri iMX6ULL Aster

imx: add support for Toradex Colibri iMX6ULL Aster

Add support for Toradex Colibri iMX6ULL 256MB module on Aster carrier
board. The Colibri iMX6ULL is a SODIMM-sized SoM based on NXP i.MX 6ULL
application processor, featuring a power-efficient Arm Cortex-A7 core.
Aster is a carrier board for the Colibri Family of Computer on Modules.
Aster carrier board has headers compatible with Arduino Uno and
Raspberry Pi (RPi) maker board, thus enabling users to use a huge
variety of third party shields/add-on boards.

Specifications of iMX6ULL:

 - CPU: i.MX 6ULL, 1x Arm Cortex-A7 (528-900MHz)
 - RAM: 256-512MB DDR3L (16 Bit)
 - Flash: 512MB SLC NAND (MX30LF4G28AC)

Specifications of Aster carrier board:

 - USB 2.0: 2x Host, 1x Client (Shared)
 - 10/100 Mbit Ethernet (KSZ8041)
 - 1x I2C, 1x SPI, 4x PWM, 4x ADC, 1x CAN, up to 3x LEDs, up to 39 GPIOs
 - Audio out: 1x (Stereo), Audio in: 1x (Stereo)
 - 1x Camera Parallel Interface, 4-wire Resistive Touch
 - Battery backed M41T0M6 RTC module
 - JTAG

Flashing instructions:

 1. Boot initramfs image from U-Boot

    setenv bootfile openwrt-imx-cortexa7-toradex_colibri-imx6ull-aster-initramfs-fit-zImage.itb
    run setup && tftpboot ${kernel_addr_r} ${bootfile} && bootm ${kernel_addr_r}

 2. Download openwrt-imx-cortexa7-toradex_colibri-imx6ull-aster-squashfs-sysupgrade.tar to device
 3. Run sysupgrade

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