Correct the bcm47xx watchdog option.  The convention of bcm watchdogs is
the _WDT suffix.

Fixes: 8dace304 ("ARM: multi_v7_defconfig: Enable BCM47xx/BCM5301x drivers")
Signed-off-by: Valentin Rothberg <valentinrothberg@gmail.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>