[PATCH ] leds: add new transient trigger for one shot timer support

[Shuah Khan]

This patch adds a new transient trigger for one shot timer support and is
to be used for the following example use cases:

- Control of vibrate (phones, tablets etc.) hardware by userspace app.
- Use of LED by userspace app as activity indicator.
- Use of LED by userspace app as a kind of watchdog indicator -- as
       long as the app is alive, it can keep the LED illuminated, if it dies
       the LED will be extinguished automatically.
- Use by any userspace app that needs a transient GPIO output

Transient trigger exports two attributes:
       transient_enabled -     one shot timer enable mechanism.
                               1 when enabled, 0 when disabled.
                               enabled state indicates a timer
                               with a value of transient_time running.
                               disabled state indicates no active timer
                               running.
       transient_time -        one shot timer value. When transient_enabled
                               is set, transient_time value is used to start
                               a timer that runs once.
       When timer expires transient_enabled goes back to disabled state,
       transient_time is left at the set value to be used when transient
       is enabled at a future time. This will allow user app to set the
       time once and enable it to run it once for the specified value as
       needed.