sunxi: provide default DRAM config for sun50i in Kconfig

sunxi: provide default DRAM config for sun50i in Kconfig

To avoid enumerating the very same DRAM values in defconfig files
for each and every Allwinner A64 board out there, let's put some sane
default values in the Kconfig file.
Boards with different needs can override them at any time.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>