Allow target frequency to be set/modified easily.

[TCal]

I would like to change the target frequency targeting for example the default Arty A7 board.

There is a LiteX argument --sys-clk-freq <freq>, but it seems this doesn't work in CFU Playground, even when I pass it using the LITEX_EXTRA_ARGS variable.

[TCal]

False alarm -- it does work. Even when the system clock is set to a frequency other than 100MHz, there is still a 100MHz clock fed into the PLL, which generates the new clock. For example, this does work for me (in proj/proj_template_v/):

 make EXTRA_LITEX_ARGS="--sys-clk-freq=87500000" USE_SYMBIFLOW=1  prog

to get a 87.5MHz clock.