mfd: pm8921: Add charger cell

The 8921 pmic is equipped with a charger block that can trickle charge
and fast charge a battery. The charger block has 32 interrupts for
notifying various charging events. It needs some parameters such as max
and min voltage of the system, charging resume voltage.

Add code to support the charger module in the core file.

Change-Id: I082b3009dd070af4120b1cd170972e7bf88ce810
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>