power: bq27421: set fast hibernate mode at power off

To minimize a leakage after power off, fuel guage should be
enter the hibernate mode. So, set the fast hibernate mode
before device shutdown.

Change-Id: I3bf2ed3051b397a6abc2af3924ecd312723e2732
Signed-off-by: choongryeol.lee <choongryeol.lee@lge.com>