sunxi: Use H3/A64 DRAM initialization code for R40

The R40 seems to have a variant of the memory controller found in
the H3 and A64 SoCs. Adapt the code for use on the R40. The changes
are based on released DRAM code and comparing register dumps from
boot0.

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