powerpc/of-device-tree: Factor MTD physmap bindings out of booting-without-of

It's easier to find bindings descriptions in separate files.  So factor
out the MTD physmap bindings into a separate file to not clutter
booting-without-of.txt more.

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