hw/intc/loongarch_pch: Modify register name PCH_PIC_xxx_OFFSET with PCH_PIC_xxx

hw/intc/loongarch_pch: Modify register name PCH_PIC_xxx_OFFSET with PCH_PIC_xxx

Macro PCH_PIC_HTMSI_VEC_OFFSET and PCH_PIC_ROUTE_ENTRY_OFFSET is renamed
as PCH_PIC_HTMSI_VEC and PCH_PIC_ROUTE_ENTRY separately, it is easier to
understand.

Signed-off-by: Bibo Mao <maobibo@loongson.cn>
Reviewed-by: Clement Mathieu--Drif <clement.mathieu--drif@eviden.com>
Reviewed-by: Song Gao <gaosong@loongson.cn>
Message-Id: <20250507023148.1877287-3-maobibo@loongson.cn>
Signed-off-by: Song Gao <gaosong@loongson.cn>