watchdog: Add Broadcom BCM2835 watchdog timer driver

This adds a driver for watchdog timer hardware present on Broadcom BCM2835 SoC,
used in Raspberry Pi and Roku 2 devices.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Tested-by: Stephen Warren <swarren@wwwdotorg.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>
Cc: linux-rpi-kernel@lists.infradead.org
Cc: linux-watchdog@vger.kernel.org
Cc: devicetree-discuss@lists.ozlabs.org