powerpc/40x: Fix assembler warning about r0

The assembler says:
  arch/powerpc/kernel/head_40x.S:623: Warning: invalid register expression

It's objecting to the use of r0 as the RA argument. That's because
when RA = 0 the literal value 0 is used, rather than the content of
r0, making the use of r0 in the source potentially confusing.

Fix it to use a literal 0, the generated code is identical.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200722022422.825197-1-mpe@ellerman.id.au