util: Expose addr config mechanisms in the gem5 op java wrapper.

util: Expose addr config mechanisms in the gem5 op java wrapper.

These allow you to set the target physical address, and map or unmap the
region of physical memory. This is not automatic for two reasons. First,
the address needs to be configured before the mapping is done, and
there's no way to ensure that ordering when everything is handled
automatically. Second, if the user isn't going to use the address based
mechanism, then the mapping and access to /dev/mem isn't necessary and
may prevent using the other call types.

Change-Id: I0f9c32d6bfa402ba59ea1bf5672fb7f11003568d
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/28184
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>