powerpc: Add a checkpatch wrapper with our preferred settings

This makes it easy to run checkpatch with settings that I like.

Usage is eg:

  $ ./arch/powerpc/tools/checkpatch.sh -g origin/master..

To check all commits since origin/master.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Reviewed-by: Russell Currey <ruscur@russell.cc>