beagle bsp: disable watchdog on am335x

On recent u-boots, the watchdog is turned on / left enabled. The
Beaglebone Black rev. C ships with such a u-boot internally so any
application booting from it must disable the watchdog.

Therefore this change is needed to boot an RTEMS app out-of-the-box
on a BBB Rev C - otherwise the user button must be held during boot
(to bypass the stock uboot) or the internal uboot must be updated. To
allow for a better out-of-the-box experience, we just turn off the
watchdog.

c/src/lib/libcpu/arm/shared/include/am335x.h

@@ -274,3 +274,9 @@
     /* Control posted mode and functional SW reset */
 #define AM335X_TIMER_TCAR2            0x058
     /* Second captured value of counter register */
+#define AM335X_WDT_BASE                0x44E35000
+    /* Watchdog timer */
+#define AM335X_WDT_WWPS                0x34
+    /* Command posted status */
+#define AM335X_WDT_WSPR                0x48
+    /* Activate/deactivate sequence */