board: dts: rockchip: Add NanoPi M4B

Add initial support for NanoPi M4B, a minor revision of
the original NanoPi M4.

Commit details of rk3399-nanopi-m4b.dts sync from Linux 5.12-rc4:
"arm64: dts: rockchip: Add NanoPi M4B board"
(sha1: c7b03115003f7f337ab165542cee37148cf30a8a)

Signed-off-by: Alexandre Vicenzi <alexandre.vicenzi@suse.com>
Reviewed-by: Kever Yang <kever.yang@rock-cihps.com>