dt-bindings: add binding for tft displays based on ilitek,ili9486

This binding is for the tft displays based on ilitek,ili9486.
ozzmaker,piscreen and waveshare,rpi-lcd-35 are such displays

Signed-off-by: Kamlesh Gurudasani <kamlesh.gurudasani@gmail.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/8938c7a53598db793bbcddf205dcf35bcc19a18e.1581270802.git.kamlesh.gurudasani@gmail.com