video: add STM32 LTDC display controller

The STM32 LTDC display controller provides a parallel digital RGB and
signals for horizontal, vertical synchronization, Pixel Clock and Data
Enable as output to interface directly to a variety of LCD and TFT panels.

The LTDC main features are:
- 24-bit RGB Parallel Pixel Output, Programmable timings & polarity for
  HSync, VSync and Data Enable.
- 2 layers with Blending, Color Keying, Window position & size,
  Dithering, Background color, Color Look-Up Table (CLUT).
- Supported layer color formats: ARGB8888, RGB888, RGB565, ARGB1555,
  ARGB4444, L8 CLUT, AL44 & AL88

This LTDC driver:
- supports: RGB parallel output with timings & polarity, 1 layer
  in RGB565.
- supports but with hard-coded configurations: blending, window
  position & size (crop), background color.
- does not support yet: rgb888, argb8888, 8-bit clut, dithering.

This LTDC driver is compatible with all stm32 platforms with the
LTDC IP and has been tested on stm32 f746-disco board.

Signed-off-by: Philippe CORNU <philippe.cornu@st.com>