poplar: add myself as co-maintainer

Add myself as co-maintainer for Poplar board, as I'm currently
working on it (re-testing releases, addressing issues etc).

CC: Jorge Ramirez-Ortiz <jorge.ramirez.ortiz@gmail.com>
CC: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Igor Opaniuk <igor.opaniuk@foundries.io>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Jorge Ramirez-Ortiz <jorge.ramirez.ortiz@gmail.com>

board/hisilicon/poplar/MAINTAINERS

@@ -1,6 +1,7 @@
 Poplar BOARD
 M:     Jorge Ramirez-Ortiz <jorge.ramirez.ortiz@gmail.com>
 M:     Shawn Guo <shawn.guo@linaro.org>
+M:     Igor Opaniuk <igor.opaniuk@gmail.com>
 S:     Maintained
 F:     board/hisilicon/poplar
 F:     doc/board/hisilicon/poplar.rst