power: twl4030_charger: detect battery presence prior to enabling charger

TWL4030's Battery Charger seems to be designed for non-hotpluggable
batteries.

If battery is not present in the system, BATSTS is always set with the
expectation that software will take actions to move to a required safe
state (could be power down or disable various charger paths).

It does not seem possible even by manipulating the edge detection
of the event (using BCIEDR2 register) to have a consistent hotplug
handling. This seems to be the result of BATSTS interrupt generated
when the thermistor of the battery pack is disconnected from the
dedicated ADIN1 pin. Clearing the status just results in the status
being regenerated by the monitoring ADC(MADC) and disabling the
edges of event just makes hotplug no longer function. The only
other option is to disable the detection of the MADC by disabling
BCIMFEN4::BATSTSMCHGEN (battery presence detector) - but then, we can
never again detect battery reconnection.

So, detect battery presence based on precharge(which is hardware
automatic state) or default main charger configuration at the time of
probe and enable charger logic only if battery was present.

Reported-by: Russell King <linux@arm.linux.org.uk>
Tested-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>