drm/sun4i: Add a driver for the display frontend

The display frontend is an hardware block that can be used to implement
some more advanced features like hardware scaling or colorspace
conversions. It can also be used to implement the output format of the VPU.

Let's create a minimal driver for it that will only enable the hardware
scaling features.

Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Link: https://patchwork.freedesktop.org/patch/msgid/029cdc3478bf89d422f5e8d9e600baf5e48ce4db.1516613040.git-series.maxime.ripard@free-electrons.com