nvmem: Move nvmem code in dev/nvmem

[manu]

Summary

We've removed kernel option EXT_RESOURCES almost two years ago.
While it was ok to have some code under a common 'extres' subdirectory
at first, we now have a lot of consumer of it and we made it mandatory
so no need to have it under a cryptic name.

Sponsored by: Beckhoff Automation GmbH & Co. KG

[dab]

LGTM

[manu]

Remove _EXTRES_ from header guard