Add Ubiquiti EdgeRouter Lite (ERL) kernel config file

The ERL is a fairly cheap (~$100 USD) and readily available dual core
MIPS64 device so it makes a useful MIPS reference platform.

This is based in part on the kernel config generated by the mkerlimage
script from http://rtfm.net/FreeBSD/ERL/.

Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D3884