ice40: Export global networks.
ice40: Export global networks.

 * Enable pins to drive a global network though padin
 * Enable drive a global network through fabout.
 * Route clocks on the fabric (hopefully via global network?).
 * Maybe CEN/Reset will go via global network now too?
 * Seems like checker routes?

Signed-off-by: Tim 'mithro' Ansell <b1c1d8736f20db3fb6c1c66bb1455ed43909f0d8@mith.ro>

SymbiFlow/symbiflow-rr-graph