machine/rp2040: implement Flash interface

Signed-off-by: deadprogram <ron@hybridgroup.com>

src/machine/machine_rp2040_gpio.go

@@ -9,7 +9,7 @@ import (
 	"unsafe"
 )
 
-type io struct {
+type ioType struct {
 	status volatile.Register32
 	ctrl   volatile.Register32
 }
@@ -21,7 +21,7 @@ type irqCtrl struct {
 }
 
 type ioBank0Type struct {
-	io                 [30]io
+	io                 [30]ioType
 	intR               [4]volatile.Register32
 	proc0IRQctrl       irqCtrl
 	proc1IRQctrl       irqCtrl