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>
diff --git a/doc/board/rockchip/rockchip.rst b/doc/board/rockchip/rockchip.rst
index 955e685..fbb9983 100644
--- a/doc/board/rockchip/rockchip.rst
+++ b/doc/board/rockchip/rockchip.rst
@@ -61,6 +61,7 @@
      - Firefly ROC-RK3399-PC
      - FriendlyElec NanoPC-T4 (nanopc-t4-rk3399)
      - FriendlyElec NanoPi M4 (nanopi-m4-rk3399)
+     - FriendlyElec NanoPi M4B (nanopi-m4b-rk3399)
      - FriendlyARM NanoPi NEO4 (nanopi-neo4-rk3399)
      - Google Bob (chromebook_bob)
      - Khadas Edge (khadas-edge-rk3399)