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/libbsp/arm/beagle/clock.c

@@ -251,6 +251,15 @@ beagle_clock_initialize(void)
       OMAP3_TCLR_OVF_TRG | OMAP3_TCLR_AR | OMAP3_TCLR_ST);
   /* also initilize the free runnning timer */
   omap3_frclock_init();
+
+#if IS_AM335X
+  /* Disable AM335X watchdog */
+  mmio_write(AM335X_WDT_BASE+AM335X_WDT_WSPR, 0xAAAA);
+  while(mmio_read(AM335X_WDT_BASE+AM335X_WDT_WWPS) != 0) ;
+  mmio_write(AM335X_WDT_BASE+AM335X_WDT_WSPR, 0x5555);
+  while(mmio_read(AM335X_WDT_BASE+AM335X_WDT_WWPS) != 0) ;
+#endif
+
 }
 
 static void beagle_clock_at_tick(void)

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 */