mtd: OneNAND: add enable / disable methods to onenand_chip

Add enable / disable methods called from get_device() / release_device().
These can be used, for example, to allow the driver to prevent the voltage
regulator from being put to sleep while OneNAND is in use.

Signed-off-by: Adrian Hunter <adrian.hunter@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>