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/board/rockchip/evb_rk3399/MAINTAINERS b/board/rockchip/evb_rk3399/MAINTAINERS
index 3b9d60e..25e308d 100644
--- a/board/rockchip/evb_rk3399/MAINTAINERS
+++ b/board/rockchip/evb_rk3399/MAINTAINERS
@@ -49,6 +49,12 @@
 F:	configs/nanopi-m4-2gb-rk3399_defconfig
 F:	arch/arm/dts/rk3399-nanopi-m4-2gb-u-boot.dtsi
 
+NANOPI-M4B
+M:	Alexandre Vicenzi <linux@alxd.me>
+S:	Maintained
+F:	configs/nanopi-m4b-rk3399_defconfig
+F:	arch/arm/dts/rk3399-nanopi-m4b-u-boot.dtsi
+
 NANOPI-NEO4
 M:	Jagan Teki <jagan@amarulasolutions.com>
 S:	Maintained