power/mt8183: Hold PMIC enable to force S5->G3 transition.
power/mt8183: Hold PMIC enable to force S5->G3 transition.

Asserting VSYSSNS should only be done as a last measure. If the
PMIC is configured properly, it will shut down upon holding
POWER+HOME key for 8+ seconds. This is shorter than the S5->G3
timeout, so we should never need to assert VSYSSNS.

BRANCH=none
BUG=b:134912821
TEST=reboot ap-off, immediately powerb, see that AP turns on and
     stays on.
TEST=apshutdown, PMIC shuts off gracefully
TEST=In power/common.c, change S5_INACTIVITY_TIMEOUT to 3 seconds,
     see that state machines forces PMIC off using VSYSSNS.
TEST=Boot DUT: mosys eventlog clear; poweroff
     power on DUT, run dut-control power_state:off
     Wait 8 seconds for EC to go to G3, power on DUT again
     mosys eventlog list shows events with correct time stamps:
0 | 2019-06-14 15:44:31 | Log area cleared | 79
1 | 2019-06-14 15:44:38 | System boot | 0
2 | 2019-06-14 15:44:38 | Chrome OS Developer Mode
3 | 2019-06-14 15:45:13 | System boot | 0
4 | 2019-06-14 15:45:13 | Chrome OS Developer Mode

Change-Id: I9b73d06e07296e47e15fe87dd87fffac2af04d12
Signed-off-by: Nicolas Boichat <f3daba1cdc403799935674d3cd4b0e5bd1804232@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/1660073
Reviewed-by: Daisuke Nojiri <fd5f93af191bf7e8f73ea71cc3c0f66b41b1dd49@chromium.org>

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