uboot-mediatek: rework support for Bananapi BPi-R64 board

Provide U-Boot variants for SD-card as well as eMMC boot, so we can
generate whole-disk images for the device.
While at it, rename 'mt7622' to 'mt7622-rfb1' to make it less confusing
now that more boards are being added.

Thanks to Frank Wunderlich (@frank-w) for making that nice SVG image
explaining the MMC boot process[1] and for providing the necessary
binary header blobs.

[1]: https://github.com/frank-w/BPI-R64-ATF


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