Fix Physical Memory to allow memory sizes bigger than 128MB.

Fix Physical Memory to allow memory sizes bigger than 128MB.
Kinda port DRAM to new memory system. The code is *really* ugly (not my fault) and right now something about the stats it uses
causes a simulator segfault.

src/SConscript:
    Add dram.cc to sconscript
src/mem/physical.cc:
src/mem/physical.hh:
    Add params struct to physical memory, use params, make latency function be virtual
src/python/m5/objects/PhysicalMemory.py:
    Add DRAMMemory python class

--HG--
extra : convert_revision : 5bd9f2e071c62da89e8efa46fa016f342c01535d