Added external RAM FIFO to u2plus.
Added code branch to ext_fifo.v using generate that instantiates
different input and out fifo's and touched nobl_fifo code so that it
works at 18 and 36bit widths.
Added 2nd DCM to top level to generate off chip RAMCLK.
Added explicit I/O instances to top level for tristate drivers and
changed signals to core as needed.
Creted new FIFO's in core gen to replace much larger FIFO's used on u2rev3