Fix for missing supervisor mode when running on E51 (#96)

The E51 core on the U54-MC lacks supervisor mode, thus the plic_s_ie and plic_s_thresh are NULL when running on this core. This adds checks for this case.

machine/minit.c

@@ -124,10 +124,17 @@ static void hart_plic_init()
     return;
 
   size_t ie_words = plic_ndevs / sizeof(uintptr_t) + 1;
-  for (size_t i = 0; i < ie_words; i++)
-    HLS()->plic_s_ie[i] = ULONG_MAX;
+  for (size_t i = 0; i < ie_words; i++) {
+     if (HLS()->plic_s_ie) {
+        // Supervisor not always present
+        HLS()->plic_s_ie[i] = ULONG_MAX;
+     }
+  }
   *HLS()->plic_m_thresh = 1;
-  *HLS()->plic_s_thresh = 0;
+  if (HLS()->plic_s_thresh) {
+      // Supervisor not always present
+      *HLS()->plic_s_thresh = 0;
+  }
 }
 
 static void wake_harts()