No Ready/Valid signal from holdover board

The current interface to the holdover board includes the following control/status nets (wrt expansion board): TIME_VALID_i, CLOCK_STATUS_o, HOCLK_IN_USE_i.

CLOCK_STATUS_o is only asserted when TIME_VALID_i is deasserted and the ICPT-1 is ready to enter holdover mode. However, it can take up to 45 mins for the ICPT-1 to be ready to enter holdover mode from when TIME_VALID_i is asserted, and there is currently no line to indicate that the device is ready to enter holdover. Therefore with the current implementation, the switch can only know if the holdover clock is valid when it deasserts TIME_VALID, i.e. at the point it needs to use the clock.

Proposed solution is to add a valid/ready signal from the holdover board.