arch: Make MMU::flushAll virtual

This is enabling ISA specific MMUs to reimplement the flushing
according to their TLB structure

Change-Id: Ic407ab95137b299206cb94926fb69d8898ed33f8
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/45779
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/arch/generic/mmu.hh

@@ -61,7 +61,7 @@ class BaseMMU : public SimObject
     }
 
   public:
-    void
+    virtual void
     flushAll()
     {
         dtb->flushAll();