powerpc: Enforce usage of RA 0-R31 where possible

Some macros use RA where when RA=R0 the values is 0, so make this
the enforced mnemonic in the macro.

Idea suggested by Andreas Schwab.

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