hw/arm/realview: Specify explicitly the GIC has 64 external IRQs

hw/arm/realview: Specify explicitly the GIC has 64 external IRQs

When not specified, Cortex-A9MP configures its GIC with 64 external
IRQs (see commit a32134aad89 "arm:make the number of GIC interrupts
configurable"). Add the GIC_EXT_IRQS definition (with a comment)
to make that explicit.

Except explicitly setting a property value to its same implicit
value, there is no logical change intended.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20250212154333.28644-4-philmd@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>