stm32/i2c: Fully support peripheral I2C4.

ports/stm32/i2c.c

@@ -650,6 +650,10 @@ STATIC mp_obj_t pyb_i2c_make_new(const mp_obj_type_t *type, size_t n_args, size_
         } else if (strcmp(port, MICROPY_HW_I2C3_NAME) == 0) {
             i2c_id = 3;
         #endif
+        #ifdef MICROPY_HW_I2C4_NAME
+        } else if (strcmp(port, MICROPY_HW_I2C4_NAME) == 0) {
+            i2c_id = 4;
+        #endif
         } else {
             nlr_raise(mp_obj_new_exception_msg_varg(&mp_type_ValueError,
                 "I2C(%s) doesn't exist", port));

ports/stm32/i2c.h

@@ -42,6 +42,7 @@ typedef struct _pyb_i2c_obj_t {
 extern I2C_HandleTypeDef I2CHandle1;
 extern I2C_HandleTypeDef I2CHandle2;
 extern I2C_HandleTypeDef I2CHandle3;
+extern I2C_HandleTypeDef I2CHandle4;
 extern const mp_obj_type_t pyb_i2c_type;
 extern const pyb_i2c_obj_t pyb_i2c_obj[4];