1. add ISR handler apis so that users of different layers can hook their own isr handler on different GPIO.
Audio project has different software layers, they need different gpio isr handler for layer instead of processing all GPIO interrupts in one handler.
If this kind of calling a handler from isr is not proper, please kindly point out.
2. add gpio example code.
3. improve gpio.rst
4. add readme for gpio example
Squashed commits:
[278e50f] update: GPIO
1. coding style, add a space between conditional or loop keyword and an opening paren.
2. modify some return value and doc
3. use printf in example code
Squashed commits:
[efb23bb] minor change of comment
