linux-yocto/5.10: cfg: fix ppc64 configuration warnings

Recent tweaks to the kern tools triggered some warnings that were
previously hidden.

With these two commits, ppc64 has a clean audit:

    c0cfc016a51 qemuppc64: fix 5.10 configuration warnings
    489c76aa9fb qemu-ppc64.cfg: Do not set CONFIG_POWER4

Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>