Log Message:
add a u-boot for pinebook pro.  it's mostly functional though reboot
is not working for me currently, and the power LED stays off until
netbsd boots still.

this is based upon u-boot 2020.01-rc5 with patches from:

https://patchwork.ozlabs.org/project/uboot/list/?series=142716

which add specific pinebook pro support.  (these sources are known
to have a working reboot, but not from a build i've made.)