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

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

ports/rp2/rp2_pio.c

@@ -360,6 +360,10 @@ STATIC const mp_rom_map_elem_t rp2_pio_locals_dict_table[] = {
     { MP_ROM_QSTR(MP_QSTR_SHIFT_LEFT), MP_ROM_INT(0) },
     { MP_ROM_QSTR(MP_QSTR_SHIFT_RIGHT), MP_ROM_INT(1) },
 
+    { MP_ROM_QSTR(MP_QSTR_JOIN_NONE), MP_ROM_INT(0) },
+    { MP_ROM_QSTR(MP_QSTR_JOIN_TX), MP_ROM_INT(1) },
+    { MP_ROM_QSTR(MP_QSTR_JOIN_RX), MP_ROM_INT(2) },
+
     { MP_ROM_QSTR(MP_QSTR_IRQ_SM0), MP_ROM_INT(0x100) },
     { MP_ROM_QSTR(MP_QSTR_IRQ_SM1), MP_ROM_INT(0x200) },
     { MP_ROM_QSTR(MP_QSTR_IRQ_SM2), MP_ROM_INT(0x400) },