powerpc/powernv: Don't clobber r9 in relative_toc()

With OPAL, r8 and r9 will be used to pass the OPAL base and entry
for debugging purposes (those informations are also in the
device-tree). We don't want to clobber those registers that
early.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>