teensy/teensy_hal: Use MP_REGISTER_ROOT_POINTER().

This uses MP_REGISTER_ROOT_POINTER() to register pyb_stdio_uart and removes
the same from mpconfigport.h.

Signed-off-by: David Lechner <david@pybricks.com>

ports/teensy/mpconfigport.h

@@ -41,9 +41,6 @@ extern const struct _mp_obj_module_t pyb_module;
 
 #define MP_STATE_PORT MP_STATE_VM
 
-#define MICROPY_PORT_ROOT_POINTERS \
-    struct _pyb_uart_obj_t *pyb_stdio_uart; \
-
 // type definitions for the specific machine
 
 #define UINT_FMT "%u"

ports/teensy/teensy_hal.c

@@ -77,3 +77,5 @@ void mp_hal_gpio_clock_enable(GPIO_TypeDef *gpio) {
 void extint_register_pin(const void *pin, uint32_t mode, int hard_irq, mp_obj_t callback_obj) {
     mp_raise_NotImplementedError(NULL);
 }
+
+MP_REGISTER_ROOT_POINTER(struct _pyb_uart_obj_t *pyb_stdio_uart);