This resolves various warnings and critical warnings from Vivado.
In particular, the asynchronous loops in the xilinx hardware RNG were
giving a lot of critical warnings, which proved to be difficult to
suppress, so this instead makes all the xilinx platforms use the
'nonrandom.vhdl' implementation, which always returns an error.
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>
