ARM: bcm2835: implement machine restart hook

Implement the machine restart hook using the SoC's watchdog timer module.
To support this, define a DT binding for the watchdog module, and add it
to the device tree.

The downstream rpi-split branch contains a full watchdog timer driver
implementation, which also implements the restart hook. However, the
restart function is largely separate from the watchdog driver, so for
simplicity, the restart hook is implemented here directly in the main
machine source file.

Overall structure (separate setup/restart) functions derived from the
picoxcell ARM support.

Watchdog register IO sequence taken from code by Simon Arlott. Note that
the watchdog module is not documented in BCM2835-ARM-Peripherals.pdf.

Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>