ARCv2: IDU-intc: Use build registers for getting numbers of interrupts

This enhancement is needed to allow masking all available common interrupts
in IDU interrupt controller in boot time since the kernel can
discover a number of them from the build register. Also now there
is no need to specify in device tree a list of used core interrupts
by IDU. E.g. before:

    idu_intc: idu-interrupt-controller {
        compatible = "snps,archs-idu-intc";
        interrupt-controller;
        interrupt-parent = <&core_intc>;
        #interrupt-cells = <2>;
        interrupts = <24 25 26 27 28 29 30 31>;
    };

and after:

    idu_intc: idu-interrupt-controller {
        compatible = "snps,archs-idu-intc";
        interrupt-controller;
        interrupt-parent = <&core_intc>;
        #interrupt-cells = <2>;
    };

Signed-off-by: Yuriy Kolerov <yuriy.kolerov@synopsys.com>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>