litedram: Add an L2 cache with store queue

This adds a cache between the wishbone and litedram with the following
features (at this point, it's still evolving)

  - 128 bytes line width in order to have a reasonable amount of
litedram pipelining on the 128-bit wide data port.

  - Configurable geometry otherwise

  - Stores are acked immediately on wishbone whether hit or miss
(minus a 2 cycles delay if there's a previous load response in the
way) and sent to LiteDRAM via 8 entries (configurable) store queue

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

Makefile

@@ -50,7 +50,7 @@ core_files = decode_types.vhdl common.vhdl wishbone_types.vhdl fetch1.vhdl \
 	loadstore1.vhdl mmu.vhdl dcache.vhdl writeback.vhdl core_debug.vhdl \
 	core.vhdl
 
-soc_files = wishbone_arbiter.vhdl wishbone_bram_wrapper.vhdl \
+soc_files = wishbone_arbiter.vhdl wishbone_bram_wrapper.vhdl sync_fifo.vhdl \
 	wishbone_debug_master.vhdl xics.vhdl syscon.vhdl soc.vhdl
 
 soc_sim_files = sim_console.vhdl sim_uart.vhdl sim_bram_helpers.vhdl \