sun6i: Add LCD display support for MSI Primo81 tablet

The MSI Primo81 tablet has B079XAN01/LP079X01 7.85" 768x1024 IPS
MIPI display, connected to the parallel LCD interface via SSD2828
bridge chip. The panel has 18-bit color depth and needs dithering,
in spite of having RGB data delivered from A31s to SSD2828 using
24-bit arrangement.

Signed-off-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>
Acked-by: Anatolij Gustschin <agust@denx.de>
Acked-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>