arm64: add a std.ec2 config

Mostly to document basic harware present on the platform; knowing that
Graviton exposes an ns8250 uart alone is quite helpful.

Reviewed by: andrew, imp, manu
Seems accurate: cperciva
Sponsored by: NetApp, Inc.
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D36776