[U-Boot] Support for the Raspberry Pi 3B+?

[Qwerty Chouskie]

The defconfig for the 3 seems to work fine, except that for some reason 
Ethernet doesn't work (from what I understand, it is on the USB bus, 
though u-boot doesn't detect it?

A quick google found some (maybe) useful files (e.g. the DTS) here: 
https://www.spinics.net/lists/arm-kernel/msg646251.html

It would be nice to have an explicit defconfig for the 3B+ for peace of 
mind, even if it is basically the same as the 3.

-QwertyChouskie