power: smb135x-charger: introduce thermal control

Add support for userspace to set thermal levels. These levels translate
to input current limits on USB or disabling drawing any current from
both USB and DC in extreme cases. Use SYSTEM_TEMP_LEVEL property for
userspace to notify the desired thermal level. Upon receiving the
thermal level set the limits or enable/disable charging based on
previous and new level.

Change-Id: I29e48f37daa354821e50dc92c3a1ca81040d2e32
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>