buildroot

DEVELOPERS: Add myself to mpv & others

Additionally added myself to ffmpeg, rpi-firmware and rpi-userland as
changes to mpv may require updates or other changes to these packages
and vice-versa.

Signed-off-by: Mahyar Koshkouei <mahyar.koshkouei@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>