stm32: Add interface description for ltdc controller. This defines the...

stm32: Add interface description for ltdc controller.  This defines the interface to perform hardware accelerated layer operation by the ltdc controller.

The following methods must be supported by the implementation:

- gevideoinfo
- getplaneinfo
- getlid
- setclut
- getclut
- setcolor
- getcolor
- setcolorkey
- getcolorkey
- setalpha
- getalpha
- setblendmode
- getblendmode
- setarea
- getarea
- update

And if DMA2D is supported:

- blit
- blend

The method up_ltdcgetlayer provides access to a reference of a specific ltdc
layer.

Signed-off-by: Marco Krahl <ocram.lhark@gmail.com>