riscv: build starfive JH7110 DTBs

These are known to work if loaded manually by loader(8) (for VF2 at
least). If nothing else, it is useful to provide a DTB closely tied to
the kernel version.

Approved by: re (cperciva)
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D53117

(cherry picked from commit 96b137c044d8f7266757e95ff134119f3ac51be5)
(cherry picked from commit 2257adb848d0d932f31419b93a2e04ba60b28936)