ARM: dts: msm: Add cooling device property to charger in PMI8998

Add cooling-cells property to PMI8998 charger. This enables the charger
cooling device to be referenced in a thermal zone definition.

Change-Id: I8128a131ab0e56658bb261ac457378713108a3b2
Signed-off-by: Ram Chandrasekar <rkumbako@codeaurora.org>