Add an option to select which SoCs we are building for. It is intended to
be used with any SoC specific drivers, for example a ThunderX nic driver
would use something like the following in files.arm64:

arm64/cavium/thunder_nic.c optional soc_cavm_thunderx thndr_nic

Reviewed by: imp
Sponsored by: ABT Systems Ltd
Differential Revision: https://reviews.freebsd.org/D3479