mvebu: new subtarget cortex A53

mvebu: new subtarget cortex A53

This commit introduces new subtarget for Marvell EBU Armada Cortex A53
processor based devices.

The first device is Globalscale ESPRESSObin. Some hardware specs:

SoC: Marvell Armada 3700LP (88F3720) dual core ARM Cortex A53
     processor up to 1.2GHz
RAM: 512MB, 1GB or 2GB DDR3
Storage: SATA interface
         µSD card slot with footprint for an optional 4GB EMMC
         4MB SPI NOR flash for bootloader
Ethernet: Topaz Networking Switch (88E6341) with 3x GbE ports
Connectors: USB 3.0
            USB 2.0
            µUSB port connected to PL2303SA (USB to serial bridge
            controller) for UART access
Expansion: 2x 46-pin GPIO headers for accessories and shields with
           I2C, GPIOs, PWM, UART, SPI, MMC, etc
           MiniPCIe slot
Misc: Reset button, JTAG interface

Currently booting only from µSD card is supported.
The boards depending on date of dispatch can come with various U-Boot
versions. For the newest version 2017.03-armada-17.10 no manual
intervention should be needed to boot OpenWrt image. For the older ones
it's necessary to modify default U-Boot environment:

 1. Interrupt boot process to run U-Boot command line,

 2. Run following commands:
    (for version 2017.03-armada-17.06 and 2017.03-armada-17.08)
     setenv bootcmd "load mmc 0:1 0x4d00000 boot.scr; source 0x4d00000"
     saveenv

    (for version 2015.01-armada-17.02 and 2015.01-armada-17.04)
     setenv bootargs "console=ttyMV0,115200 root=/dev/mmcblk0p2 rw rootwait"
     setenv bootcmd "ext4load mmc 0:1 ${fdt_addr} armada-3720-espressobin.dtb; ext4load mmc 0:1 ${kernel_addr} Image; booti ${kernel_addr} - ${fdt_addr}"
     saveenv

 3. Poweroff, insert SD card with OpenWrt image, boot and enjoy.

Signed-off-by: Tomasz Maciej Nowak <tomek_n@o2.pl>