test/py: return a RAM address different from 0 as it can be interpreted as NULL

Some functions test that the given address is not NULL (0) and fail or
have a different behaviour if that's the case (e.g. hexport_r).

Let's make the RAM base address to be not zero by setting it to 2MiB if
that's the case.

2MiB is chosen because it represents the size of an ARM LPAE/v8 section.

Suggested-by: Stephen Warren <swarren@wwwdotorg.org>
Signed-off-by: Quentin Schulz <quentin.schulz@bootlin.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Stephen Warren <swarren@nvidia.com>
Tested-by: Stephen Warren <swarren@nvidia.com>