hw/intc/loongson_ipi: Move IPICore structure to loongson_ipi_common.h

hw/intc/loongson_ipi: Move IPICore structure to loongson_ipi_common.h

Move the IPICore structure and corresponding common fields
of LoongsonIPICommonState to "hw/intc/loongson_ipi_common.h".

Signed-off-by: Bibo Mao <maobibo@loongson.cn>
[PMD: Extracted from bigger commit, added commit description]
Co-Developed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Bibo Mao <maobibo@loongson.cn>
Tested-by: Bibo Mao <maobibo@loongson.cn>
Acked-by: Song Gao <gaosong@loongson.cn>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Tested-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Message-Id: <20240805180622.21001-7-philmd@linaro.org>