power: qti_battery_charger: Block PMIC GLINK Tx for debug battery

The remote subsystem does not transmit PMIC GLINK notifications
triggered by charging parameter changes when a debug battery (one whose
battery ID is set to 7.5 Kohms) is detected. This allows it to enter low
power mode provided no queries for individual power supply (PSY)
properties are made of it.

Power consumption monitoring tests require it to enter low power mode
and stay there, which can happen only if it is not disturbed by having
to respond to queries made by HLOS for PSY properties. This change
enables the user to block all PMIC GLINK Tx messages from this driver to
remote subsystem by setting the boolean debugfs property "block_tx".

Once this is done, all subsequent reads of PSY properties will return
not the instantaneous values of those quantities, but the last (stale)
values obtained during the last read that was made prior to Tx being
blocked. To return to normal behaviour and obtain instantaneous values
from the remote subsystem, the debugfs property must be cleared.

Change-Id: Ie9b6ec78dcb8ec00102e9d7d23e238445045206f
Signed-off-by: Guru Das Srinagesh <gurus@codeaurora.org>