common/button: Add support for HW_REINIT button combo
common/button: Add support for HW_REINIT button combo

If volume up + volume down + power buttons are held down for 30
seconds, then indicate to AP that HW_REINIT is requested by the
user. Since recovery mode is triggered by holding down the three buttons
for 10 seconds, button_check_hw_reinit_required checks for 20 more
seconds only.

BUG=b:37682514
BRANCH=None
TEST=Verified that:
1. If volume up, volume down and power are held for 30 seconds, AP
sees the hw reinit request.
2. If either of the buttons is released before 30 seconds, AP does not
see the hw reinit request.

Change-Id: I3a53966dcdcb84d73f160578411b6c0f62225b95
Signed-off-by: Furquan Shaikh <6be5c96c464778f89679b1062a2e0a6af93d2aa5@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/487002
Reviewed-by: Aaron Durbin <af0622ab041e8b0f8e5819838dcc93898196e71f@chromium.org>
Reviewed-by: Randall Spangler <62698fdbb84d1779579ee80c3f39fac22017e5bc@chromium.org>
Reviewed-by: Nicolas Boichat <f3daba1cdc403799935674d3cd4b0e5bd1804232@chromium.org>

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