link: Enable +5VALW at boot and leave it enabled

This works around a problem where disabling +5VALW glitches +3VALW_EC,
which may cause the EC to brown out or hang.

BUG=chrome-os-partner:16600
BRANCH=link
TEST=manual

1. When the system boots, look for this line as the first x86 power state:
  [0.004977 x86 power state 0 = G3, in 0x0001]
2. Boot the system.  Should boot normally.
3. Shut down the system using the power button.
4. After ~10 seconds, you should see that line of output again.
5. At the EC console: 'gpioget enable_5valw' should output:
  1* ENABLE_5VALW

This should ideally be combined with a hardware fix to add 30+ ms of
delay to EC_EN_5V, since when the EC is reset via power+refresh that
tri-states EC_EN_5V, and it takes ~22ms for the EC to boot and start
driving EC_EN_5V again.

Change-Id: Iba4d961d064105faf988a35c2277e9d7406e39e2
Signed-off-by: Randall Spangler <rspangler@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/39334
Reviewed-by: Yung-Chieh Lo <yjlou@chromium.org>

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