MDX: Exposed P4IO Buffer Filling Mode to Spicecfg (#463)

## Description of change
Extension of #460 
An option was exposed to `spicecfg` (`P4IOBufferMode`) for toggling
between the Thread and Backfill buffer fill implementations from the
prior PR.

## Testing
- Verified the option properly toggled between the two implementations

src/spice2x/acio/mdxf/mdxf.h

@@ -3,6 +3,13 @@
 #include "../module.h"
 
 namespace acio {
+    
+    typedef enum {
+        THREAD_MODE = 0,
+        BACKFILL_MODE = 1
+    } MDXFBufferFillMode;
+
+    extern MDXFBufferFillMode MDXF_BUFFER_FILL_MODE;
 
     class MDXFModule : public ACIOModule {
     public: