regulator: fixed: fix regulator-fixed-clock

For regulator-fixed-clock, the device's private data is never set so in
fixed_clock_regulator_set_enable() is null and the function cannot
complete successfully.

Rename the _plat structure to _priv to better represent its role and set
this as the private data.  As shown by the set_enable() function and by
using the same .of_to_plat hook as regulator-fixed, the platform data is
regulator_common_plat so also set .plat_auto correctly.

Finally, set up the private data by adding a .probe function to look up
the clock and set the member variable.

Fixes: f3b5100a ("regulator: fixed: add possibility to enable by clock")
Signed-off-by: John Keeping <jkeeping@inmusicbrands.com>
Reviewed-by: Simon Glass <sjg@chromium.org>