powerpc: Enforce usage of R0-R31 where possible

Enforce the use of R0-R31 in macros where possible now we have all the
fixes in.

R0-R31 macros are removed here so that can't be used anymore.  They
should not be defined anywhere.

Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>