clkdiv: added clkdiv

clkdiv: added clkdiv

This change allows arbitrary divider values to be used for the 6MHz FTDI
SPI/JTAG clock.

I find that, on the Lattice NX Evaluation Board, the FTDI clock divider
needs to be set to a value of 3 or higher in order to program the flash
rom. This may be because the board uses an ES (Early Silicon/Engineering
Sample) CrossLink/NX-40. I see similar behavior with the Radiant
programmer where a divisor of 2 or higher is needed.

With a slower clock divider, ecpprog is also able to verify programmed
flash rom content.