Use of QT_GSTREAMER_USE_OPENGL_PLUGIN broke video playback
at least on imx6 and imx8 boards. Move the option for now
to raspberrypi only, since that's were the original issue
was reported.

Task-number: QTBUG-78855
Change-Id: I28fc98bcdac3ca02af07bbdb0466daa9c4612df3
Reviewed-by: VaL Doroshchuk <valentyn.doroshchuk@qt.io>