rk3399: Add ROC-RK3399-PC Mezzanine board

Add Firefly ROC-RK3399-PC Mezzanine board which is an
extension board on top of roc-rk3399-pc.

Will drop the separate defconfig file, once we support
the board detection at runtime.

Signed-off-by: Suniel Mahesh <sunil@amarulasolutions.com>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
diff --git a/board/firefly/roc-pc-rk3399/MAINTAINERS b/board/firefly/roc-pc-rk3399/MAINTAINERS
index 7564dd2..68a5b75 100644
--- a/board/firefly/roc-pc-rk3399/MAINTAINERS
+++ b/board/firefly/roc-pc-rk3399/MAINTAINERS
@@ -1,6 +1,8 @@
 ROC-RK3399-PC
 M:	Levin Du <djw@t-chip.com.cn>
+M:	Suniel Mahesh <sunil@amarulasolutions.com>
 S:	Maintained
 F:	board/firefly/roc-pc-rk3399
 F:	include/configs/roc-pc-rk3399.h
 F:	configs/roc-pc-rk3399_defconfig
+F:	configs/roc-pc-mezzanine-rk3399_defconfig