ARM: bcm2835: Move the restart/power_off handling to the WDT driver

Since the WDT is what's used to drive restart and power off, it makes
more sense to keep it there, where the regs are already mapped and
definitions for them provided.  Note that this means you may need to
add CONFIG_BCM2835_WDT to retain functionality of your kernel.

Signed-off-by: Eric Anholt <eric@anholt.net>
Acked-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Lee Jones <lee.jones@linaro.org>