Kconfig: sunxi: prepare for using drivers/ram/sunxi

Kconfig: sunxi: prepare for using drivers/ram/sunxi

At the moment all Allwinner DRAM initialisation routines are stored in
arch/arm/mach-sunxi, even though those "drivers" are just a giant
collection of writel's, without any architectural dependency.

The R528/T113-s SoC (with ARM cores) and the D1/D1s Soc (with RISC-V
cores) share the same die, so should share the same DRAM init routines as
well.

To prepare for this, add a new sunxi directory inside drivers/ram, and
add some stub entries to prepare for the addition of the share DRAM code
for those SoCs.

The RISC-V D1(s) SoCs will probably use SPL_DM, so for that SoC this
would be the right directory anyway.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>