hw/arm: Move virt's PSCI DT fixup code to arm/boot.c

hw/arm: Move virt's PSCI DT fixup code to arm/boot.c

Move virt's PSCI DT fixup code to arm/boot.c and set this fixup to
happen automatically for every board that doesn't mark "psci-conduit"
as disabled. This way emulated boards other than "virt" that rely on
PSIC for SMP could benefit from that code.

Cc: Peter Maydell <peter.maydell@linaro.org>
Cc: Jason Wang <jasowang@redhat.com>
Cc: Philippe Mathieu-Daudé <f4bug@amsat.org>
Cc: Marcel Apfelbaum <marcel.apfelbaum@zoho.com>
Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: qemu-devel@nongnu.org
Cc: qemu-arm@nongnu.org
Cc: yurovsky@gmail.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>