doc: allwinner: Reorder sections, document memory mapping

Update the Allwinner platform documentation.
Reorder the section, to have the build instructions first, followed by
hints about the installation.

Add some ASCII art about the layout of our virtual memory map, which
uses a non-trivial condensed virtual address space.

Change-Id: Iaaa79b4366012394e15e4c1b26c212b5efb6ed6a
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
1 file changed