Allow RPI 3B and 3B+ to boot with the official upstream DTBs

[sylvain_sylvaingarrigues.com]

Summary

The root node of upstream RPI3 DTBs used to be compatible with brcm,brcm2836: it is not any longer. Therefore, add brcm,bcm2837 as a compatibility string to our platform and drivers.

For reference, from the device trees available at https://github.com/raspberrypi/firmware/blob/master/boot/:

bcm2710-rpi-3-b.dtb:

compatible = "raspberrypi,3-model-b", "brcm,bcm2837";
model = "Raspberry Pi 3 Model B";

bcm2710-rpi-3-b-plus.dtb

compatible = "raspberrypi,3-model-b-plus", "brcm,bcm2837";
model = "Raspberry Pi 3 Model B+";

Test Plan

Tested on RPI3B

[db]

Tested on both models of RPi-3
compatible = "raspberrypi,3-model-b", "brcm,bcm2837";
and
compatible = "raspberrypi,3-model-b-plus", "brcm,bcm2837";

bcm2835_cpufreq0: <CPU Frequency Control> on cpu0
...

bcm2835_cpufreq0: ARM 600MHz, Core 250MHz, SDRAM 400MHz, Turbo OFF

powerd failed to start without this fix now runs fine.

[gonzo]

LGTM