eeepc-wmi: Add support for T101MT Home/Express Gate key

This key is different than other hotkeys, having seperate scan
codes for press, release, and hold, so it requires some special
filtering. Press and release events are passed on, and hold events
are ignored since sparse-keymap does not support hardware
autorepeat.

Note that "Home" in the context of this button doesn't mean the
same thing as the usual Home key, and it really isn't clear at
all what is meant by "Home". The manufacurer's description of the
button indicates that it should launch some sort of touch screen
settings interface on short press and apply a desktop rotation on
long press.

Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: Corentin Chary <corentin.chary@gmail.com>
Signed-off-by: Matthew Garrett <mjg@redhat.com>