gpio_keys: fix whitespaces
gpio_keys: change msleep intervals

As suggested by yank555.lu
gpio_keys: add a semaphore

Wait until "release" event before "press"ing it again

by yank555.lu
gpio_keys: switch to use a dedicated suspended variable for flip-cover

Previous implementation cannot take actions properly
when flip-covers are attached on or off quickly.

Since KEY_POWER can be actually pressed again
while screen is getting turned off or on,

Add a flip_cover_suspended variable to take actions properly.