powerpc: Move remaining .c files from arch/ppc64 to arch/powerpc

This also deletes the now-unused Makefiles under arch/ppc64.

Both of the files moved over could use some merging, but for now I
have moved them as-is and arranged for them to be used only in 64-bit
kernels.  For 32-bit kernels we still use arch/ppc/kernel/idle.c and
drivers/char/generic_nvram.c as before.

Signed-off-by: Paul Mackerras <paulus@samba.org>