mtd: Add orion_nand devicetree bindings

Allow a NAND chip using the orion_nand driver to be described using devicetree.

Changes since last submission (V4) [Addressing comments by]:-
* WARN when bank-width is out of range [Andrew Lunn]

Changes since last submission (V3):-
* Document all parameters [Grant Likely]
* Convert bank-width to be in bytes
* Add explicit defaults for cle, ale and bank-width

Signed-off-by: Jamie Lentin <jm@lentin.co.uk>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>