video: sunxi: Hook up SSD2828 with the sunxi video driver

Convert GPIO names from Kconfig strings into pin numbers for
the 'ssd2828_config' struct. Add SSD2828 initialization between
enabling the parallel LCD interface and turning on the backlight.

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