Input: tps65218-pwrbutton - add support for tps65217 variant

The power button found in tps65217 device is very similar to the tps65218,
so let's enhance the driver to support both variants.

This driver enables us to use tps65217's power button as KEY_POWER on
am335x boards (directly connected button in chiliboard, accessible pin
via expansion header in beaglebone). This patch has been tested with
chiliboard.

Signed-off-by: Marcin Niestroj <m.niestroj@grinn-global.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>