mem: Add tRTP to the DRAM controller

This patch adds the tRTP timing constraint, governing the minimum time
between a read command and a precharge. Default values are provided
for the existing DRAM types.

src/mem/dram_ctrl.hh

@@ -486,6 +486,7 @@ class DRAMCtrl : public AbstractMemory
     const Tick tRP;
     const Tick tRAS;
     const Tick tWR;
+    const Tick tRTP;
     const Tick tRFC;
     const Tick tREFI;
     const Tick tRRD;