powerpc/boot: Only build OPAL code when necessary

Only build the OPAL console code in when necessary. This looks like it
should use CONFIG_PPC_POWERNV, but because the opal-call.S code is
64-bit only, we must only build it when we're building the boot
wrapper 64-bit.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>