powerpc: Merge arch/ppc64/kernel/vio.c into arch/powerpc/kernel/vio.c

Merge arch/ppc64/kernel/vio.c into arch/powerpc/kernel/vio.c, update
the Makefiles to make it work, and make ARCH=ppc64 still work.

Michael's version put vio.c in arch/powerpc/sysedv but after consolting
Paulus, this one puts it in arch/powerpc/kernel.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>