Rockchip: rename output plugin name to libv4l-encplugin.so

Plugin has to be dynamically opened in chromium sandbox.
The name needs to be hard-coded in sandbox and libv4l.
To avoid changing both places for every new platform,
using the same name for encoder plugin would be better.

BUG=chromium:405861
TEST=test on pinky and make sure plugin is called successfully.

Change-Id: Ib76afdf8384db6a0f3cdd8734cdcc81b6be14e90
Reviewed-on: https://chromium-review.googlesource.com/240128


Reviewed-by: Wu-cheng Li <wuchengli@chromium.org>
Commit-Queue: Heng-ruey Hsu <henryhsu@chromium.org>
Tested-by: Heng-ruey Hsu <henryhsu@chromium.org>