cpu/rocket, soc_sdram: Connect mem_axi to LiteDRAM, bypass WB bus

cpu/rocket, soc_sdram: Connect mem_axi to LiteDRAM, bypass WB bus

Connect Rocket's dedicated port for cached RAM accesses (mem_axi)
directly to the LiteDRAM data port, bypassing the shared LiteX
(Wishbone) bus.

When both Rocket's mem_axi and LiteDRAM's port have the same data
width, use a native point-to-point AXI connection.

Otherwise, convert both ends to Wishbone, and use the Wishbone
data width converter to bridge the gap.
FIXME: In the future, this part should be replaced with a native
AXI data width converter!

Signed-off-by: Gabriel Somlo <gsomlo@gmail.com>