Missing I/O timing information

[ed-xmos]

Should probably be in guide and certainly in appnote. I/O timing in code appears to only correct at 50MHz. For other clock rates, it needs to be modified. Example timing calculations attached. xcore sdram timing.xlsx

[ed-xmos]

THis is now partially addressed Timing setup is done automatically by simply selecting a divider. Some timing calculations can be found in server.xc (done for slicekit) It would be good to add this to the docs, ideally an app-note.