hw/arm/bcm2836: Simplify use of 'reset-cbar' property

bcm2836_realize() is called by

 - bcm2836_class_init() which sets:

    bc->cpu_type = ARM_CPU_TYPE_NAME("cortex-a7")

 - bcm2837_class_init() which sets:

    bc->cpu_type = ARM_CPU_TYPE_NAME("cortex-a53")

Both Cortex-A7 / A53 have the ARM_FEATURE_CBAR set. If it isn't,
then this is a programming error: use &error_abort.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20231123143813.42632-3-philmd@linaro.org>