man4: move allwinner pages and logic to a standard place

They are shared by arm, arm64, and now riscv. Note that they are
always installed by default, where MAN_ARCH=all.

MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D52585

(cherry picked from commit 15c79c6fa60809a9dc627e5051704cf2a556c6db)