of: use kbasename instead of open coding

Several places in DT code open code the equivalent of kbasename.
Replace them.

The behavior for root nodes in node_name_cmp will be slightly different.
Instead of comparing "/", "" will be compared. The comparison will be
the same.

Reviewed-by: Andy Shevchenko <andy.shevhchenko@gmail.com>
Signed-off-by: Rob Herring <robh@kernel.org>