[tools] Add rtc-clear tool

Added rtc-clear tool. It can be used for clearing any pending
RTC wake up events. E.g. before powering off a device you can clear
the RTC events to make sure device does not wake up right after power
off to that event.

The tool is based on the rtc_clear function in dsmetool.c of DSME project.
Since DSME is LGPL licensed, a new license file LICENSE.rtc-clear was
added.

Signed-off-by: Kalle Jokiniemi <kalle.jokiniemi@jolla.com>