sunxi: fix 32MB load address layout

The default load addresses for the various payloads (kernel, DT,
ramdisk) on systems with just 32MB of DRAM have some issues:
For a start the preceding comment doesn't match the actual values:
apparently they were copied from the 64MB S3 layout, then halved, but
since 0x5 is NOT the half of 0x10, they don't match up.
Also those projected maximum sizes are quite restrictive: it's not easy
to build a compressed kernel image with just 4MB. The only defconfig in
mainline Linux that supports the F1C100s (the only 32MB user so far)
creates a 6MB compressed / 15MB uncompressed kernel.
Rearrange the default load addresses to accommodate such a kernel: we
allow an 7MB/16MB kernel, and up to 5MB of ramdisk, stuffing the smaller
binaries like the DTB towards the end, just before the relocated U-Boot.
Shrink the size for DTB and scripts on the way, there is no need for
allowing up to 512K for them.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Jernej Skrabec <jernej.skrabec@gmail.com>