drm: Add support for Amlogic Meson Graphic Controller

The Amlogic Meson Display controller is composed of several components :

DMC|---------------VPU (Video Processing Unit)----------------|------HHI------|
   | vd1   _______     _____________    _________________     |               |
D  |-------|      |----|            |   |                |    |   HDMI PLL    |
D  | vd2   | VIU  |    | Video Post |   | Video Encoders |<---|-----VCLK      |
R  |-------|      |----| Processing |   |                |    |               |
   | osd2  |      |    |            |---| Enci ----------|----|-----VDAC------|
R  |-------| CSC  |----| Scalers    |   | Encp ----------|----|----HDMI-TX----|
A  | osd1  |      |    | Blenders   |   | Encl ----------|----|---------------|
M  |-------|______|----|____________|   |________________|    |               |
___|__________________________________________________________|_______________|

VIU: Video Input Unit
---------------------

The Video Input Unit is in charge of the pixel scanout from the DDR memory.
It fetches the frames addresses, stride and parameters from the "Canvas" memory.
This part is also in charge of the CSC (Colorspace Conversion).
It can handle 2 OSD Planes and 2 Video Planes.

VPP: Video Post Processing
--------------------------

The Video Post Processing is in charge of the scaling and blending of the
various planes into a single pixel stream.
There is a special "pre-blending" used by the video planes with a dedicated
scaler and a "post-blending" to merge with the OSD Planes.
The OSD planes also have a dedicated scaler for one of the OSD.

VENC: Video Encoders
--------------------

The VENC is composed of the multiple pixel encoders :
 - ENCI : Interlace Video encoder for CVBS and Interlace HDMI
 - ENCP : Progressive Video Encoder for HDMI
 - ENCL : LCD LVDS Encoder
The VENC Unit gets a Pixel Clocks (VCLK) from a dedicated HDMI PLL and clock
tree and provides the scanout clock to the VPP and VIU.
The ENCI is connected to a single VDAC for Composite Output.
The ENCI and ENCP are connected to an on-chip HDMI Transceiver.

This driver is a DRM/KMS driver using the following DRM components :
 - GEM-CMA
 - PRIME-CMA
 - Atomic Modesetting
 - FBDev-CMA

For the following SoCs :
 - GXBB Family (S905)
 - GXL Family (S905X, S905D)
 - GXM Family (S912)

The current driver only supports the CVBS PAL/NTSC output modes, but the
CRTC/Planes management should support bigger modes.
But Advanced Colorspace Conversion, Scaling and HDMI Modes will be added in
a second time.

The Device Tree bindings makes use of the endpoints video interface definitions
to connect to the optional CVBS and in the future the HDMI Connector nodes.

HDMI Support is planned for a next release.

Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>