LoongArch: KVM: Use 64-bit register definition for EIOINTC

LoongArch: KVM: Use 64-bit register definition for EIOINTC

With in-kernel emulated eiointc driver, hardware register can be
accessed by different size, there is reg_u8/reg_u16/reg_u32/reg_u64
union type with EIOINTC register.

Here use 64-bit type with register definition and remove union type
since most registers are accessed with 64-bit method. And this makes
EIOINTC emulated driver simpler.

Signed-off-by: Bibo Mao <maobibo@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>