Fix Linux task preemption on Versatile board

Backport from master:

  Recent versions of the Linux kernel will not preempt CPU-intensive
  tasks unless the clock used by sched_clock() works.  On -M versatilepb
  that's the 24MHz timer in the system controller.  It's a very simple
  timer, so implement it.

Signed-off-by: Daniel Jacobowitz <dan@codesourcery.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>

hw/arm_sysctl.c

@@ -7,6 +7,8 @@
  * This code is licenced under the GPL.
  */
 
+#include "hw.h"
+#include "qemu-timer.h"
 #include "sysbus.h"
 #include "primecell.h"
 #include "sysemu.h"
@@ -71,8 +73,7 @@ static uint32_t arm_sysctl_read(void *opaque, target_phys_addr_t offset)
     case 0x58: /* BOOTCS */
         return 0;
     case 0x5c: /* 24MHz */
-        /* ??? not implemented.  */
-        return 0;
+        return muldiv64(qemu_get_clock(vm_clock), 24000000, ticks_per_sec);
     case 0x60: /* MISC */
         return 0;
     case 0x84: /* PROCID0 */