panfrost: Initial stub for Panfrost driver

This patch adds an initial stub for the Gallium driver, containing
simple screen functions and the majority of the driver headers but no
actual functionality. It further adds the winsys glue for linking in
this stub driver via kmsro on Rockchip/Amlogic boards.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Acked-by: Jason Ekstrand <jason@jlekstrand.net>
Acked-by: Rob Clark <robdclark@gmail.com>
Acked-by: Eric Anholt <eric@anholt.net>
Acked-by: Emil Velikov <emil.velikov@collabora.com>