power: pm8921-charger: increase hearbeat if battery is low

When the battery percent charge goes low we want to increase the
frequency of state of charge updates. This helps the correction
algorithm ample opportunities to correct the state of charge and
prevent a low voltage shutdown.

Change-Id: Ie846350ee4c1fd70198d28646779b20349134ef7
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>