rp2/rp2_pio: Add fifo_join support for PIO.

The PIO state machines on the RP2040 have 4 word deep TX and RX FIFOs.  If
you only need one direction, you can "merge" them into either a single 8
word deep TX or RX FIFO.

We simply add constants to the PIO object, and set the appropriate bits in
`shiftctrl`.

Resolves #6854.

Signed-off-by: Tim Radvan <tim@tjvr.org>

examples/rp2/pio_uart_rx.py

@@ -22,6 +22,7 @@ PIO_RX_PIN = Pin(3, Pin.IN, Pin.PULL_UP)
     autopush=True,
     push_thresh=8,
     in_shiftdir=rp2.PIO.SHIFT_RIGHT,
+    fifo_join=PIO.JOIN_RX,
 )
 def uart_rx_mini():
     # fmt: off