Input: add powerkey driver for HISI 65xx SoC

This driver provides a input driver for the power button on the
HiSi 65xx SoC for boards like HiKey.

This driver was originally by Zhiliang Xue <xuezhiliang@huawei.com>
then basically rewritten by Jorge, but preserving the original
module author credits.

Signed-off-by: Jorge Ramirez-Ortiz <jorge.ramirez-ortiz@linaro.org>
[jstultz: Reworked commit message, folded in other fixes/cleanups
from Jorge, implemented some larger cleanups suggested by DmitryT]
Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>