powerpc/boot: Fix build failure in 32-bit boot wrapper

OPAL is not callable from 32-bit mode and the assembly code for it
may not even build (depending on how binutils was configured).

References: https://buildd.debian.org/status/fetch.php?pkg=linux&arch=powerpcspe&ver=4.8.7-1&stamp=1479203712


Fixes: 656ad58e ("powerpc/boot: Add OPAL console to epapr wrappers")
Cc: stable@vger.kernel.org # v4.8+
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>