reef: Enable battery learn-mode when battery charging not allowed
reef: Enable battery learn-mode when battery charging not allowed

When the battery charging is not allowed and charging at high load
audible noise is observed from the BD9995X charger. To prevent this
issue enable the battery-learn mode when battery charging is not
allowed.

This audible noise is related to the fact that in light load (<450mA
being withdrawn from VSYS) the DCDC of the charger operates
intermittently i.e. DCDC switches continuously and then stops to
regulate the output voltage and current, and sometimes to prevent
reverse current from flowing to the input. This causes a slight
voltage ripple on VSYS that falls in the audible noise frequency
(single digit kHz range). This small ripple generates audible noise
in the output ceramic capacitors (caps on VSYS and any input of DCDC
under VSYS).

BUG=chrome-os-partner:56695
BRANCH=none
TEST=When battery charging not allowed, battery is put in battery
     learn-mode & audible noise is not observed.

Change-Id: Ia22779fe4cf70dd9dd4f799a9698264e44c4c7d2
Signed-off-by: Vijay Hiremath <006c712879a3af08bfdb5c2b8491a8b2d5fd73cd@intel.com>
Reviewed-on: https://chromium-review.googlesource.com/412382
Commit-Ready: Vijay P Hiremath <006c712879a3af08bfdb5c2b8491a8b2d5fd73cd@intel.com>
Tested-by: Vijay P Hiremath <006c712879a3af08bfdb5c2b8491a8b2d5fd73cd@intel.com>
Reviewed-by: Shawn N <5259333ce42e3ace6a73988cec7115ed9d3a661a@chromium.org>

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