sunxi: add Kconfig option for the maximum accessible DRAM

Allwinner 64-bit SoCs can use 4GiB DRAM chip, however their memory map
has only allocated 3GiB for DRAM, so only 3GiB of the DRAM is
accessible.

Add a Kconfig option for the maximum accessible DRAM.

For A80 it should be a much higher value (8GiB), but as I have no A80
device to test and originally U-Boot only supports 2GiB DRAM on A80, it
currently still falls under the 2GiB situation.

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>