sandbox: Change the range used for memory-mapping tags

sandbox: Change the range used for memory-mapping tags

Sandbox keeps a table of addresses which map to pointers which are
outside its emulated DRAM. The current range from 10000000 conflicts
with the PCI range, meaning that if PCI mapping is on, that particular
address can be decoded by PCI instead of the table.

Fix this by moving the range up to the top of memory. Update the docs
while we are here.

Signed-off-by: Simon Glass <sjg@chromium.org>