bcm28xx: fix symbol name, make reboot work on raspberry pi's

target/linux/config/Config.in.watchdog

@@ -15,7 +15,7 @@ config ADK_KERNEL_CS5535_MFGPT
 config ADK_KERNEL_CS5535_CLOCK_EVENT_SRC
 	bool
 
-config ADK_KERNEL_BCM2708_WDT
+config ADK_KERNEL_BCM2835_WDT
 	bool "Hardware Watchdog for BCM28XX boards"
 	select ADK_KERNEL_WATCHDOG
 	depends on ADK_TARGET_BOARD_BCM28XX