mtd: OneNAND: OMAP2/3: prevent regulator sleeping while OneNAND is in use

Prevent OneNAND's voltage regulator from going to sleep while
OneNAND is in use, by explicitly enabling and disabling the
regulator as appropriate.

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