dt-bindings: interrupt-controller: loongson,liointc: Fix dtbs_check warning for reg-names

As we know, the Loongson-2K0500 is a single-core CPU, and the core1-
related register (isr1) does not exist. So "reg" and "reg-names" should
be set to "minItems 2"(main nad isr0).

This fixes dtbs_check warning:

DTC_CHK arch/loongarch/boot/dts/loongson-2k0500-ref.dtb
arch/loongarch/boot/dts/loongson-2k0500-ref.dtb: interrupt-controller@1fe11400: reg-names: ['main', 'isr0'] is too short
        From schema: Documentation/devicetree/bindings/interrupt-controller/loongson,liointc.yaml
arch/loongarch/boot/dts/loongson-2k0500-ref.dtb: interrupt-controller@1fe11400: Unevaluated properties are not allowed ('reg-names' was unexpected)
        From schema: Documentation/devicetree/bindings/interrupt-controller/loongson,liointc.yaml
arch/loongarch/boot/dts/loongson-2k0500-ref.dtb: interrupt-controller@1fe11400: reg: [[0, 534844416, 0, 64], [0, 534843456, 0, 8]] is too short
        From schema: Documentation/devicetree/bindings/interrupt-controller/loongson,liointc.yaml
arch/loongarch/boot/dts/loongson-2k0500-ref.dtb: interrupt-controller@1fe11440: reg-names: ['main', 'isr0'] is too short
        From schema: Documentation/devicetree/bindings/interrupt-controller/loongson,liointc.yaml

Acked-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Binbin Zhou <zhoubinbin@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>