Fixing the missing "cpu" devclass on RPi

[Peter Jeremy]

On the RPi, the "cpu" devclass is created by dev/ofw/ofw_cpu.c and depends
on the presence of a "cpus" block in the FDT.  This exists in the FreeBSD
trees (eg bcm2835-rpi-b-plus.dtb) but not in the Linux FDT populated by the
latest firmware.  This means that (eg) bcm2835_cpufreq never gets probed[1].

Injecting "cpus" via a fdt overlay was suggested but I can't get that to
work.  I have rpi-cpus.dtso as follows:
=====
/dts-v1/;
/plugin/;

/ {
	compatible = "brcm,bcm2835";
};

&{/} {
	cpus {
		cpu at 0 {
			compatible = "arm,1176jzf-s";
			device_type = "cpu";
		};
	};
};
=====
When I compile it and de-compile it, the result looks sane:
====
$ dtc -I dtb -O dts rpi-cpus.dtbo
/dts-v1/;

/  {

        compatible = "brcm,bcm2835";
        fragment at 0 {

                target-path = "/";
                __overlay__ {

                        cpus {

                                cpu at 0 {

                                        compatible = "arm,1176jzf-s";
                                        device_type = "cpu";
                                };
                        };
                };
        };
        __symbols__ {

        };
};
====

I've added rpi-cpus.dtbo to fdt_overlays in loader.conf and the boot
messages report that it's being loaded but the "cpus" section is still
missing from hw.fdt

I presume I've got something wrong in my dtso but I'm not sure what -
I haven't found a great deal of documentation on writing overlays and
the above looks correct according to what I've found.

[1] There's a separate bug in bcm2835_cpufreq.c so it'll never attach,
    but that's just a matter of fixing the compatible list.

-- 
Peter Jeremy
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