[new port] u-boot-beagleplay-a53

[oh]

Summary

u-boot for beagleplay A53 core - it might be renamed later and add support for the other TI AM6 series of SoC

Patches originates from here
https://openbeagle.org/beagleboard/u-boot.git

There might be possible to make just one u-boot-beagleplay port that builds both the r5 and the a53 and skip the D46731