poplar: add a co-maintainer for poplar board

Add myself as co-maintainer for poplar board, as I'm actively working
on the board.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
diff --git a/board/hisilicon/poplar/MAINTAINERS b/board/hisilicon/poplar/MAINTAINERS
index 0cc01c8..622e5cb 100644
--- a/board/hisilicon/poplar/MAINTAINERS
+++ b/board/hisilicon/poplar/MAINTAINERS
@@ -1,5 +1,6 @@
 Poplar BOARD
 M:     Jorge Ramirez-Ortiz <jorge.ramirez-ortiz@linaro.org>
+M:     Shawn Guo <shawn.guo@linaro.org>
 S:     Maintained
 F:     board/hisilicon/poplar
 F:     include/configs/poplar.h