Platform: OLPC: Add a regulator for the DCON

All OLPC ECs are able to turn the power to the DCON on an off. Use the
regulator framework to expose the functionality.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>