Input: rt5120 - add power key support

Add RT5120 PMIC power key support.

Signed-off-by: ChiYuan Huang <cy_huang@richtek.com>
Link: https://lore.kernel.org/r/1660100142-32493-4-git-send-email-u0084500@gmail.com
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>