include: adds interface description for ili9341

This adds the description of the generic interface to communicate with
the ili9341 lcd single chip driver used by any displays.

The interface have to be implemented by the platform specific subdriver.
The following functions must be exported:

-select
 Select the spi device before starting an operations.

-deselect
 Deselect the spi device after operations finished and if the device was
 selected before.

-sendcmd
 Send any command to the ili9341 display driver.

-sendparam
 Send any parameter corresponding to the ili9341 display driver.

-recvparam
 Receive any parameter from the ili9341 display driver. This is only possible
 for the read commands supported by the ili9341.

-sendgram
 Send pixel data to the gram of the display. This i similar to the function
 sendparam, but pixel data operations needs another handling of how to send the
 data to the display.

-recvgram
 Receive pixel data to the gram of the display. This i similar to the function
 recvparam, but pixel data operations needs another handling of how to receive
 the data from the display.

-backlight
 Change the backlight level of the display.

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