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

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

wr_board_pkg: declare eb_ethernet_slave as a component so that compiler doesn't complain when EB repo is not included

Compilation and synthesis run at any new push. Simulation run according
to CI schedule configured on OHWR.