watchdog: Add a watchdog driver for Raspberry Pi boards

This driver supports the bcm2835 watchdog found on
Raspberry Pi boards.
It is derived from the Linux driver and was tested
on two Raspberry Pi board versions (B+ and 3B+).

Signed-off-by: Etienne Dublé <etienne.duble@imag.fr>
Reviewed-by: Stefan Roese <sr@denx.de>