ESP32 in revision0 and revision1 uses touchpad to provide
current to oscillate xtal 32k. But revision2 and revision3
do not need to do that.
Note: touchpad can not work and toupad/ULP wakeup sources
are not available when toupad provides current to xtal 32k
1. add sens_struct.h
2. add definition of RTCCNTL and RTCIO
3. modify touch pad examples
4. update example code.
5. add comments add option in menuconfig
6. fix issue that pad index 8 and 9 are mismatched
7. add touch_pad_read_filtered() api to get value filtered by iir filter
8. modify touch pad isr func
9. Make the items in perihperal.ld in the sequence of address
10. delete Kconfig for touch pad
11. add touchpad filter APIs to adjust the filter
12. add touch_pad into index.rst
13. add touch_pad in Doxyfile
14. add touch_pad.rst
* Use "example" in all example function & variable names,
ie use i2c_example_xxx instead of i2c_xxx for example functions.
Closes#198https://github.com/espressif/esp-idf/issues/198
* Mark example functions, etc. static
* Replace uses of "test" & "demo" with "example"
* Split the UART example into two
* Rename "main" example files to end with "_main.c" for disambiguation
