sunxi: Provide defaults for R40 DRAM settings

These values were taken from the Banana Pi M2 Ultra fex file
found in the released vendor BSP. This is the only publicly
available R40 device at the time of this writing.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>