hw/cpu/arm_mpcore: Remove default values for GIC external IRQs

hw/cpu/arm_mpcore: Remove default values for GIC external IRQs

Implicit default values are often hard to figure out, better
be explicit. Now that all boards explicitly set the number of
GIC external IRQs, remove the default values (displaying an
error message if it is out of range).

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20250212154333.28644-9-philmd@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>