ports: Provide mp_hal_stdio_poll for sys.stdio polling where needed.

ports/pic16bit/pic16bit_mphal.c

@@ -25,6 +25,7 @@
  */
 
 #include <string.h>
+#include "py/stream.h"
 #include "py/mphal.h"
 #include "board.h"
 
@@ -51,6 +52,14 @@ void mp_hal_set_interrupt_char(int c) {
     interrupt_char = c;
 }
 
+uintptr_t mp_hal_stdio_poll(uintptr_t poll_flags) {
+    uintptr_t ret = 0;
+    if ((poll_flags & MP_STREAM_POLL_RD) && uart_rx_any()) {
+        ret |= MP_STREAM_POLL_RD;
+    }
+    return ret;
+}
+
 int mp_hal_stdin_rx_chr(void) {
     for (;;) {
         if (uart_rx_any()) {