powerpc/device-tree: Document MTD nodes with multiple "reg" tuples

Add binding for mtd nodes with multiple reg tuples.  Multiple reg tuples
are used when the flash region covers multiple devices of the same type,
but not necessarily the same size.

Signed-off-by: Stefan Roese <sr@denx.de>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>