memory: mvebu-devbus: Remove unused variable

This variable is not being used anywhere and it's only forgotten
garbage that should have been removed in the previous commit:

  commit 9b6e4c0a
  Author: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
  Date:   Fri Jul 26 10:17:38 2013 -0300

  memory: mvebu-devbus: Remove address decoding window workaround

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>