mraa_adv_func.h: Added pwm initialization replace

current code in pwm does not work for beaglebone src/pwm/pwm.c Implemented
check for pwm_init_replace

Signed-off-by: Michael Ring <mail@michael-ring.org>
Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>

src/pwm/pwm.c

@@ -161,6 +161,10 @@ mraa_pwm_read_duty(mraa_pwm_context dev)
 mraa_pwm_context
 mraa_pwm_init(int pin)
 {
+    if (advance_func->pwm_init_replace != NULL) {
+        return advance_func->pwm_init_replace(pin);
+    }
+
     if (advance_func->pwm_init_pre != NULL) {
         if (advance_func->pwm_init_pre(pin) != MRAA_SUCCESS)
             return NULL;