powerpc/of-bindings: Don't support linux,<modalias> "compatible" values

Compatible property values in the form linux,<modalias> is not documented
anywhere and using it leaks Linux implementation details into the device
tree data (which is bad).  Remove support for compatible values of this
form.

If any platforms exist which depended on this code (and I don't know of
any), then they can be fixed up by adding legacy translations to the
lookup table in this file.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>