[POWERPC] Rename device_is_compatible to of_device_is_compatible

This is more consistent and gets us closer to the Sparc code.

We add a device_is_compatible define for compatibility during the
change over.

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>