parallella/fpga: Fix timing warning
parallella/fpga: Fix timing warning

WARNING: [Vivado 12-2489] -period contains time 3.333333 which will be
rounded to 3.333 to ensure it is an integer multiple of 1 picosecond
[/home/olaj/Projects/Adapteva/oh/src/parallella/fpga/paral
lella_timing.xdc:1]

olajep/oh,olajep/oh,olajep/oh,olajep/oh