Input: introduce palmas-pwrbutton

Many palmas family of PMICs have support for interrupt based power button.
This allows the device to notify the processor of external push button
events over the shared palmas interrupt. However, this event is generated
only during a "press" operation. Software is supposed to poll(sigh!) for
detecting a release event.

The PMIC also supports ability to power off independent of the software
decisions when the button is pressed for a long duration if the PMIC is
appropriately configured on the platform.

Even though the function is similar to twl4030_pwrbutton, it is
substantially different in operation to belong to a new driver of it's own.

Based on original work done by Girish S Ghongdemath <girishsg@ti.com>

Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>