nuttx: Register PWM device only if device file is not present

If configured from menuconfig, device inode is already there
there is no need to register it again.

It was observed on STM32F7 using a config with those options:

```
CONFIG_STM32F7_TIM1=y
CONFIG_STM32F7_TIM1_PWM=y
```

Relate-to: rzr/webthing-iotjs#3
Forwarded: https://github.com/pando-project/iotjs/pull/
Change-Id: I6134f15ead43babe3e5de6a9f0d185f6629f62d2
IoT.js-DCO-1.0-Signed-off-by: Philippe Coval p.coval@samsung.com

src/modules/nuttx/iotjs_module_pwm-nuttx.c

@@ -89,13 +89,15 @@ bool iotjs_pwm_open(iotjs_pwm_t* pwm) {
     return false;
   }
 
-  struct pwm_lowerhalf_s* pwm_lowerhalf =
-      iotjs_pwm_config_nuttx(timer, pwm->pin);
+  if (access(path, F_OK) != 0) {
+    struct pwm_lowerhalf_s* pwm_lowerhalf =
+        iotjs_pwm_config_nuttx(timer, pwm->pin);
 
-  DDDLOG("%s - path: %s, timer: %d\n", __func__, path, timer);
+    DDDLOG("%s - path: %s, timer: %d\n", __func__, path, timer);
 
-  if (pwm_register(path, pwm_lowerhalf) != 0) {
-    return false;
+    if (pwm_register(path, pwm_lowerhalf) != 0) {
+      return false;
+    }
   }
 
   // File open