scripts: size_report: Fix ram_report and rom_report with anytree 2.9.0

The anytree package has introduced a breaking change in version 2.9.0 by
adding a 'size' property to the NodeMixin class. Since the TreeNode class
in size_report derives from NodeMixin and defines an attribute with the
same name, an AttributeError is raised when generating reports.
With this change, the attributes of the TreeNode class are prefixed with
an underscore to resolve the name collision and to prevent future name
collisions.

Fixes #60213.

Signed-off-by: Christian Marx <c.marx@vega.com>