common/button: Ensure debug mode exits on timeout
common/button: Ensure debug mode exits on timeout

In all states other than STATE_NONE, ensure that there is a deferred
call set after DEBUG_TIMEOUT so that debug mode exits if there is no
user activity.

BUG=b:64436180
BRANCH=None
TEST=make -j buildall. Also, verified following:

1. All combos still work fine
2. No user input for 10 seconds after entering debug mode:

++[42.318691 DEBUG MODE: Active!]
++[53.288057 DEBUG MODE: Exit!]

3. No user input for 10 seconds on sysrq path:

++[95.675863 DEBUG MODE: Active!]
+[97.200743 Button 'Volume Down' was pressed]
[97.526162 Button 'Volume Down' was released]
[107.528069 DEBUG MODE: Exit!]

4. No user input for 10 seconds on warm reset path:
+[149.374805 DEBUG MODE: Active!]
[150.863902 Button 'Volume Up' was pressed]
[151.193353 Button 'Volume Up' was released]
[161.199244 DEBUG MODE: Exit!]

5. Volup+Voldn still held down 10 seconds after entering debug mode:

+[213.704770 DEBUG MODE: Active!]
[223.705368 DEBUG MODE: Exit!]

Change-Id: I8a0aa448d31b4a746c7af2519ac6f61838bc9b99
Signed-off-by: Furquan Shaikh <6be5c96c464778f89679b1062a2e0a6af93d2aa5@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/603991
Reviewed-by: Aaron Durbin <af0622ab041e8b0f8e5819838dcc93898196e71f@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec