AXI stream is transferred exactly on the rising edge of the clock. Use
the current value of the signals for this, instead of past values.
Simulate a slower slave to ensure this is tested.
Fixes: a549fca ("Add UART receive module")
Signed-off-by: Sean Anderson <seanga2@gmail.com>
Add the recieve half of the UART. It's more or less the inverse of the
transmit half, except we manage the state explicitly. I originally did
this in hopes that yosys would recode the FSM, but it doesn't like the
subtraction in the D* states. I left in the async reset anyway since it
reduces the LUT count.
Signed-off-by: Sean Anderson <seanga2@gmail.com>