platform/xilinx: split platform for vivado to a separate file not to break ise build with unknown primitives

wr_gthe3_phy_family7: assert rx_enc_err when the link is down to reliably force link down event in the PCS

wr_gtx_phy_kintex7_lp: fixed DMTD configuration to match new clock frequencies, more control over resets

wr_si57x_interface: latch the current RFREQ value before writing it to the device (prevents changes of the RFREQ while I2C is still busy)