sbsa-ref: allow to use Cortex-A53/57/72 cpus

sbsa-ref: allow to use Cortex-A53/57/72 cpus

Trusted Firmware now supports A72 on sbsa-ref by default [1] so enable
it for QEMU as well. A53 was already enabled there.

1. https://review.trustedfirmware.org/c/TF-A/trusted-firmware-a/+/7117

Signed-off-by: Marcin Juszkiewicz <marcin.juszkiewicz@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20201120141705.246690-1-marcin.juszkiewicz@linaro.org
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>