hw/intc/loongson_ipi: Implement IOCSR address space for MIPS

hw/intc/loongson_ipi: Implement IOCSR address space for MIPS

Implement IOCSR address space get functions for MIPS/Loongson CPUs.

For MIPS/Loongson without IOCSR (i.e. Loongson-3A1000), get_cpu_iocsr_as
will return as null, and send_ipi_data will fail with MEMTX_DECODE_ERROR,
which matches expected behavior on hardware.

Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20240508-loongson3-ipi-v1-3-1a7b67704664@flygoat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>