sunxi: H616: Enable full 4GB of DRAM

The H616 is our first supported Allwinner SoC which goes beyond the 4GB
address space "barrier", by having more than 32 address bits.

Lift the preliminary 3GB DRAM limit for the H616, and update the page
table setup on the way, to actually map that last GB as well.

As not all devices are actually capable of dealing with more than 32
bits (the DMA in the EMAC for instance), we also limit U-Boot's own
DRAM usage to 4GB on the way.

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