regulator: fix sysfs name collision between dummy and fixed dummy regulator

When regulator_register_fixed() is being used to register fixed dummy
regulator, the following line will be seen in the boot log.  And the
sysfs entry for fixed dummy regulator is not shown.

  dummy: Failed to create debugfs directory

The patch renames the fixed dummy supply to "fixed-dummy" to avoid
the name collision with dummy regulator.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>