ARM: bcm2835: add USB controller to device tree

The BCM2835 SoC contains a DWC2 USB controller. Add this to the DT.

Set up the pin controller to fully enable the USB controller on the
Raspberry Pi. The GPIO setup works because the default output value for
GPIO 6 (LAN_RUN/n_reset) just happens to be 1, which enables the
USB/LAN chip.

Note that you'll need a U-Boot which enables power to the USB controller;
search for U-Boot patch "ARM: rpi_b: power on SDHCI and USB HW modules".

Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>
Signed-off-by: Olof Johansson <olof@lixom.net>