sunxi: use 6MHz PLL_VIDEO step for DE2 for higher resolution LCD

DE2 SoCs can support LCDs up to 1080p (e.g. A64), and 3MHz step won't
let PLL_VIDEO be high enough for them.

Use 6MHz step for PLL_VIDEO when using DE2, to satisfy 1080p LCD.

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Signed-off-by: Vasily Khoruzhick <anarsoul@gmail.com>
Tested-by: Vasily Khoruzhick <anarsoul@gmail.com>
Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>
Acked-by: Jagan Teki <jagan@openedev.com>