examples: cleanup examples directory

Following changes are done as a part of cleanup:

1. Moved the platform specific `C` examples from top level examples/
directory to platform/ subdirectory and renamed helloedison.c to
gpio_edison.c

2. C specific examples are moved to a new c/ subdirectory. As a part
of this process, examples are modified to follow same standards
and few new examples are also added.

3. Include the newly added C examples to relevant API documentation

4. Ran clang-format for all source files in c/, c++/, platform/
subdirectories

Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>

api/mraa/aio.h

@@ -30,7 +30,7 @@
  * AIO is the anlog input & output interface to libmraa. It is used to read or
  * set the voltage applied to an AIO pin.
  *
- * @snippet analogin_a0.c Interesting
+ * @snippet aio.c Interesting
  */
 
 #ifdef __cplusplus

api/mraa/gpio.h

@@ -33,7 +33,7 @@
  * through sysfs), or memory mapped IO via a /dev/uio device or /dev/mem
  * depending again on the board configuration.
  *
- * @snippet gpio_read6.c Interesting
+ * @snippet gpio.c Interesting
  */
 
 #ifdef __cplusplus

api/mraa/i2c.h

@@ -32,7 +32,7 @@
  * communicate to multiple i2c slaves by configuring the address.
  * @htmlinclude i2c.txt
  *
- * @snippet i2c_HMC5883L.c Interesting
+ * @snippet i2c_hmc5883l.c Interesting
  */
 
 #ifdef __cplusplus

api/mraa/iio.h

@@ -67,7 +67,7 @@ typedef struct {
  *
  * An iio context represents an IIO device
  *
- * @snippet iio_driver.c Interesting
+ * @snippet iio.c Interesting
  */
 
 #ifdef __cplusplus

api/mraa/led.h

@@ -31,6 +31,7 @@
  * LED is the Light Emitting Diode interface to libmraa. It is used to
  * access the on board LED's via sysfs.
  *
+ * @snippet led.c Interesting
  */
 
 #ifdef __cplusplus

api/mraa/pwm.h

@@ -33,7 +33,7 @@
  * of resolution so make sure you check the board & pin you are using before
  * hand.
  *
- * @snippet cycle-pwm3.c Interesting
+ * @snippet pwm.c Interesting
  */
 
 #ifdef __cplusplus

api/mraa/spi.h

@@ -34,7 +34,7 @@
  * more information on spidev devices is available
  * [here](https://www.kernel.org/doc/Documentation/spi/spidev).
  *
- * @snippet spi_mcp4261.c Interesting
+ * @snippet spi.c Interesting
  */
 
 #ifdef __cplusplus