regmap: debugfs: "registers" & "access" for single register maps

When a map covers a single register, max_register is equal
to 0, so the "registers" & "access" files were not created.
Now they will be, as register 0 must be readable for such
map to make sense.

Signed-off-by: Pawel Moll <pawel.moll@arm.com>
Signed-off-by: Mark Brown <broonie@linaro.org>