driver: ledc - update fading functions, add example and doc

1. add fading functions.
2. clear up ledc.c and ledc.h
3. update api doc.
4. add ledc example
5. edit ledc.rst and add readme for example code.
6. add s_ prefix for static global values.
7. add mutex for fade functions
8. minor changes according to the comments.

docs/api/ledc.rst

@@ -4,20 +4,19 @@ LED Control
 Overview
 --------
 
-`Instructions`_
+The LED control module is primarily designed to control the intensity of LEDs, although it can be used to generate PWM signals for other purposes as well. 
+It has 16 channels which can generate independent waveforms that can be used to drive e.g. RGB LED devices. For maximum flexibility, the high-speed as well 
+as the low-speed channels can be driven from one of four high-speed/low-speed timers. The PWM controller also has the ability to automatically increase or 
+decrease the duty cycle gradually, allowing for fades without any processor interference.
 
 Application Example
 -------------------
 
-`Instructions`_
+LEDC change duty cycle and fading control example: `examples/29_ledc <https://github.com/espressif/esp-idf/tree/master/examples/29_ledc>`_.
 
 API Reference
 -------------
 
-`Instructions`_
-
-.. _Instructions: template.html
-
 Header Files
 ^^^^^^^^^^^^
 
@@ -38,6 +37,11 @@ Macros
 .. doxygendefine:: LEDC_APB_CLK_HZ
 .. doxygendefine:: LEDC_REF_CLK_HZ
 
+Type Definitions
+^^^^^^^^^^^^^^^^
+
+.. doxygentypedef:: ledc_isr_handle_t
+
 Enumerations
 ^^^^^^^^^^^^
 
@@ -49,6 +53,16 @@ Enumerations
 .. doxygenenum:: ledc_channel_t
 .. doxygenenum:: ledc_timer_bit_t
 
+Structures
+^^^^^^^^^^
+
+.. doxygenstruct:: ledc_channel_config_t
+    :members:
+
+.. doxygenstruct:: ledc_timer_config_t
+    :members:
+
+
 Functions
 ^^^^^^^^^
 
@@ -67,3 +81,9 @@ Functions
 .. doxygenfunction:: ledc_timer_pause
 .. doxygenfunction:: ledc_timer_resume
 .. doxygenfunction:: ledc_bind_channel_timer
+.. doxygenfunction:: ledc_set_fade_with_step
+.. doxygenfunction:: ledc_set_fade_with_time
+.. doxygenfunction:: ledc_fade_func_install
+.. doxygenfunction:: ledc_fade_func_uninstall
+.. doxygenfunction:: ledc_fade_start
+