Clocks: clock & slow_clock need to be constrained

[vrytikov]

They are not constrained at ToT and time analysis is not performed for them

[aananko]

JTAG clock is not constrained also.
The pull request is for xilinx variant: 5ae559a

Also, there is a nexus4_ddr/display_clock, but I'm not sure if it makes sence to constrain it for 763 Hz frequency.