Add initial Raspberry Pi 3 support

RPI3 kernel config builds kernel compatible with latest upstream device
tree and firmware: https://github.com/raspberrypi/firmware/tree/master/boot
As of today it's 597c662a613df1144a6bc43e5f4505d83bd748ca

Default console is PL01x, so pi3-disable-bt dt overlay should be configured
in config.txt and stock U-Boot should be patched to use proper serial port.

Yet unsupported: SMP, VCHIQ, RNG driver. RNG requires some work due to
upstream device tree incompatibility.

Multiple people contributed to this work over time: db@, loos@, manu@