hw/sh4/r2d: Convert legacy qemu_allocate_irqs() to qemu_init_irqs()

hw/sh4/r2d: Convert legacy qemu_allocate_irqs() to qemu_init_irqs()

The FPGA exposes a fixed set of IRQs. Hold them in the FPGA
state and initialize them in place calling qemu_init_irqs().

Move r2d_fpga_irq enums earlier so we can use NR_IRQS within
the r2d_fpga_t structure. r2d_fpga_init() returns r2d_fpga_t,
and we dereference irq from it in r2d_init().

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20250121182445.35309-1-philmd@linaro.org>