1. Fix the duty fade check issue reported from https://github.com/espressif/esp-idf/issues/1914
2. Fix the duty overflow issue when duty_scale is set to 1.
3. Fix the duty fade error when a fade operation is under way. We must configure a new duty setting after the previous fade operation has finished due to hardware limit.
4. Add thread-safe version APIs to set duty and fade.
5. Correct the duty range in driver.
1. Add low speed channels for LEDC module.
2. Improve fade object allocate mechanism.
3. Improve ledc example, add 2 low speed channels in example.
4. Remove debug code
5. Improve the register bit field name of slow clock.
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.
1. configure LEDC timer saparately
2. add peripher_crtl.c/.h
To enable the peripheral modules, we have to set/clear the control register in dport_reg.h.
These bits are disabled by default and they are all in a same register, so we need to add a lock on that.
3. add include esp_err.h in gpio.h
