regmap: debugfs: Disambiguate dummy debugfs file name

Since commit 9b947a13 ("regmap: use debugfs even when no device")
allows the usage of regmap debugfs even when there is no device
associated, which causes several warnings like this:

(NULL device *): Failed to create debugfs directory

This happens when the debugfs file name is 'dummy'.

The first dummy debugfs creation works fine, but subsequent creations
fail as they have all the same name.

Disambiguate the 'dummy' debugfs file name by adding a suffix entry,
so that the names become dummy0, dummy1, dummy2, etc.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Mark Brown <broonie@kernel.org>