atlys: re-add pclk constraint

Contrary to what 7b02a538765b0ddbcbbd759b33312f786b491f39 said,
it seems like the build fails when either both the clk50mbufg
*and* the plk constraint is present or both are removed.

re-adding the pclk constraint makes the build succeed with
ISE 14.7 at least...