powerpc/tools/checkpatch: Ignore DT_SPLIT_BINDING_PATCH

From what I've seen, every time this warning comes up it's bogus,
so let's ignore it.

Signed-off-by: Russell Currey <ruscur@russell.cc>
Reviewed-by: Andrew Donnellan <andrew.donnellan@au1.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>