mpv: add support for Raspberry Pi "rpi" output

mpv now compiles with the "rpi" video driver that provides hardware
acceleration on the Raspberry Pi.

Tested working on a Raspberry Pi 1 Model B+ and Zero.

Signed-off-by: Mahyar Koshkouei <mahyar.koshkouei@gmail.com>
[Thomas: drop unneeded dependency on rpi-firmware.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>