shared/tinyusb: Allow ports to use 1200bps-touch without other CDC code.

This fixes the build for some esp32 and nrf boards (for example
`ARDUINO_NANO_33_BLE_SENSE` and `ARDUINO_NANO_ESP32`) due to commit
c98789a6d8.  Changes are:

- Allow the CDC TX/RX functions in `mp_usbd_cdc.c` to be enabled
  separately to those needed for `MICROPY_HW_USB_CDC_1200BPS_TOUCH`.

- Add `MICROPY_EXCLUDE_SHARED_TINYUSB_USBD_CDC` option as a temporary
  workaround for the nrf port to use.

- Declare `mp_usbd_line_state_cb()` in a header as a public function.

- Fix warning with type cast of `.callback_line_state_changed`.

Signed-off-by: Damien George <damien@micropython.org>

ports/nrf/mpconfigport.h

@@ -335,6 +335,7 @@ long unsigned int rng_generate_random_word(void);
 #if MICROPY_HW_USB_CDC
 #include "device/usbd.h"
 #define MICROPY_HW_USBDEV_TASK_HOOK extern void tud_task(void); tud_task();
+#define MICROPY_EXCLUDE_SHARED_TINYUSB_USBD_CDC (1)
 #else
 #define MICROPY_HW_USBDEV_TASK_HOOK ;
 #endif