irqchip/loongson-eiointc: Add virt extension support

Interrupts can be routed to maximal four virtual CPUs with real HW
EIOINTC interrupt controller model, since interrupt routing is encoded
with CPU bitmap and EIOINTC node combined method. Here add the EIOINTC
virt extension support so that interrupts can be routed to 256 vCPUs in
virtual machine mode. CPU bitmap is replaced with normal encoding and
EIOINTC node type is removed, so there are 8 bits for cpu selection, at
most 256 vCPUs are supported for interrupt routing.

Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Co-developed-by: Song Gao <gaosong@loongson.cn>
Signed-off-by: Song Gao <gaosong@loongson.cn>
Signed-off-by: Bibo Mao <maobibo@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>