mediatek: rework support for BananaPi BPi-R64

**What's new**

 * Bring support for the Bananapi BPi-R64 to the level desirable for
   a nice hackable routerboard.
 * Use ARM Trusted Firmware A from source. (goodbye binary preloader)
 * Use Das U-Boot from source. (see previous commit)
 * Assemble SD-card image using OpenWrt image-commands.
   (no gen_sd_cruz_foo.sh added, this is not Raspbian)
 * Updated kernel options to support root filesystem.
 * Updated DTS to match OpenWrt LAN ports, known LEDs, buttons, ...
 * Detect root device, handle sysupgrade, config restore, ...
 * Wire up (known) LEDs and buttons in OpenWrt-fashion.
 * Build one set of images from SD-card and eMMC.
 * Hopefully provide a good example of how things can be done right
   from scratch.

**Installation and images**

 * Have an empty SD-card at hand
 * Write stuff to the card, as root (card device is /dev/mmcblkX)
   - write header, gpt, bl2, atf, u-boot and recovery kernel:
     `cat *bpi-r64-boot-sdcard.img *bpi-r64-initramfs-recovery.fit > /dev/mmcblkX`
   - rescan partitions:
     `blockdev --rereadpt /dev/mmcblkX`
   - write main system to production partition:
     `cat *bpi-r64-squashfs-sysupgrade.fit > /dev/mmcblkXp5`

 * Installation to eMMC works using SD-card bootloader via TFTP
   When running OpenWrt of SD-card, issue this to trigger installation
   to eMMC:
   `fw_setenv bootcmd run emmc_init`
   Be prepared to serve the content of bin/targets/mediatek/mt7622 on
   TFTP server address 192.168.1.254.

**What's missing**

 * The red LED is always on, probably a hardware bug.
 * AHCI (probably needs DTS changes)
 * Ship SD-card image ready with every needed for eMMC install.
 * The eMMC has a second, currently unused boot partition. This would
   be ideal to store the WiFi EEPROM and Ethernet MAC address(es).
   @sinovoip ideas?

Thanks to Thomas Hühn @thuehn for providing the hardware!

Signed-off-by: Daniel Golle <daniel@makrotopia.org>