watchdog: bcm2835_wdt: Remove unused BCM283x watchdog driver and its references

The BCM2835/2836 watchdog is not used in mainline U-Boot at all. This
patch removes the driver and its references (CONFIG_BCM2835_WDT)
completely.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Paolo Pisati <p.pisati@gmail.com>