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

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

Looking at the Zynq 7000 SoC Technical Reference Manual (UG585 v1.14)
on Appendix A: Register Details, the mpcore Interrupt Controller Type
Register (ICDICTR) has the IT_Lines_Number field read-only with value
0x2, described as:

  IT_Lines_Number

          b00010 = the distributor provides 96 interrupts,
                   64 external interrupt lines.

Add a GIC_EXT_IRQS definition (with a comment) to make the number of
GIC external IRQs 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: 20250212154333.28644-6-philmd@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>