dev: Add generic driver for buzzer

This patch adds generic support for buzzers through 3 APIs:

* void buzzer_init(void)
* void buzzer_start(unsigned int frequency)
* void buzzer_stop(void)

These APIs must be implemented by each platform.

Signed-off-by: Paulo Borges <paulo.borges@openbossa.org>
cc2540mini: Implement buzzer driver

This patch implements the platform specific functions of buzzer
driver.

Signed-off-by: Paulo Borges <paulo.borges@openbossa.org>
cc2540mini: Add buzzer example

Signed-off-by: Paulo Borges <paulo.borges@openbossa.org>