gitmodules: Use the QEMU mirror of qemu-palcode

We have a mirror of the qemu-palcode repository on
git.qemu.org; use that instead of the upstream github,
in line with our general policy of keeping and using
a mirror for submodules.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20180319131743.3885-1-peter.maydell@linaro.org