saul_reg: make ptr to device descriptor non const

sys/auto_init/saul/auto_init_gpio.c

@@ -55,7 +55,7 @@ void auto_init_gpio(void)
 
         LOG_DEBUG("[auto_init_saul] initializing GPIO #%u\n", i);
 
-        saul_reg_entries[i].dev = p;
+        saul_reg_entries[i].dev = (void *)p;
         saul_reg_entries[i].name = p->name;
         if ((p->mode == GPIO_IN) || (p->mode == GPIO_IN_PD) ||
             (p->mode == GPIO_IN_PU)) {

sys/include/saul_reg.h

@@ -38,7 +38,7 @@ extern "C" {
  */
 typedef struct saul_reg {
     struct saul_reg *next;          /**< pointer to the next device */
-    const void *dev;                /**< pointer to the device descriptor */
+    void *dev;                      /**< pointer to the device descriptor */
     const char *name;               /**< string identifier for the device */
     saul_driver_t const *driver;    /**< the devices read callback */
 } saul_reg_t;