cypress-touchkey: Attempt to filter out phantom and invalid key presses

This is an attempt to workaround a well known hardware bug of some Samsung devices.
Pratically, the high activity of the radio hardware, in particular the antenna, could cause interference on Melfas chip, which is located very close to the antenna.
This interference could induce bad interrupts that are seen by the chip as possible back or menu key presses.
These phantom key presses are happening expecially in 2G mode.

The problem and the possible workarounds are described by Electric Bill on this XDA forum thread:
http://forum.xda-developers.com/showthread.php?t=1551580

The code changes are also based on these commits made by CyanogenMod Team:
- https://github.com/CyanogenMod/android_kernel_samsung_aries/commit/4afad77c61c7cb54cce293034cdb07a928f22d20
- https://github.com/CyanogenMod/android_kernel_samsung_aries/commit/cb8fa22cba177709848321e8100ee60ee1684530
pmic8xxx-keypad: Prevent invalid HOME key presses

This is an attempt to prevent invalid HOME key presses caused by the antenna interference.
If the HOME key status doesn't change, then the key press is invalid and will be ignored.