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>