Add API for Flash devices

[aaron.nyholm at unfoldedeffective.com]

Attached is next iteration for the flashdev API. The allocating of
memory used by the region functionality of the API has been moved
to the device driver to allow different device drivers to have
a different number of maximum regions.

Other small changes like fixing spelling errors were made and better
type choice.

Thanks in advance for any feedback and time spent reviewing.

Regards,
Aaron.