mfd: pm8921: remove VBAT IRQ from charger driver control

The VBAT IRQ needs to be controlled by the pm8xxx-batt-alarm
battery alarm driver.  Remove VBAT IRQ control from the
pm8921-charger driver.

Change-Id: I478b4b572e4c08cfcd50ba4c288f847ea549f775
Signed-off-by: David Collins <collinsd@codeaurora.org>
[sboyd: Drop charger part]
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>