Support DMA-optimized fb_copyarea from the Raspberry Pi kernel

This provides basic 2D acceleration support for Raspberry Pi
to speed up moving windows and scrolling.

Signed-off-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>
Mention Raspberry Pi in the 2D acceleration section of the README

Signed-off-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>