MIPS: AR7: register watchdog device only if enabled in hw configuration

This patch checks if the watchdog enable bit is set in the DCL register
meaning that the hardware watchdog actually works and if so, register the
ar7_wdt platform_device.

Signed-off-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>