fix: assign FMC2 CLK1 with correct P and N swapping

FMC2 CLK1 has the positive side of the differential signal routed to an N input pin on the FPGA, and the negative side to a P pin. Set the pin constraints to reflect this, like it is done for FMC1 CLK1, which is also PN-swapped.

The C_FMC2_CLK_INVERTED_MASK constant allows the design to correctly assign the IBUFDS inputs and invert the IBUFDS output if necessary.

The alternative would have been to keep the pin assignments wrong with regard to the schematics and just rely on the design to invert the signal. In that case pi_fmc1_clk1_{p,n} would have had to be changed instead.