hw/arm/virt: Remove dependency on Cortex-A15 MPCore peripherals

When using a Cortex-A15, the Virt machine does not use any
MPCore peripherals. Remove the dependency.

Fixes: 7951c7b7 ("hw/arm: Express dependencies of the virt machine with Kconfig")
Reported-by: Miroslav Rezanina <mrezanin@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20201107114852.271922-1-philmd@redhat.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>