power: pm8921-charger: change BOOT_DONE configuration

Set the BOOT_TIMER_EN bit when writing the
BOOT_DONE configuration. This is required to make
sure the boot timer will be enabled during the
next phone boot.

Change-Id: I0c43de3a0c285979f7bccd42e464ca0d2fbbe42a
Signed-off-by: David Keitel <dkeitel@codeaurora.org>