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>

ports/rp2/modules/rp2.py

@@ -31,7 +31,8 @@ class PIOASMEmit:
         autopush=False,
         autopull=False,
         push_thresh=32,
-        pull_thresh=32
+        pull_thresh=32,
+        fifo_join=0
     ):
         # uarray is a built-in module so importing it here won't require
         # scanning the filesystem.
@@ -40,7 +41,8 @@ class PIOASMEmit:
         self.labels = {}
         execctrl = 0
         shiftctrl = (
-            (pull_thresh & 0x1F) << 25
+            fifo_join << 30
+            | (pull_thresh & 0x1F) << 25
             | (push_thresh & 0x1F) << 20
             | out_shiftdir << 19
             | in_shiftdir << 18