ARM: multi_v7_defconfig: Enable Broadcom Kona watchdog

In order to get rid of bcm_defconfig, enable this driver which is needed
by some Broadcom SoCs based on the Kona architecture to reboot.

Acked-by: Scott Branden <scott.branden@broadcom.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>