power_supply: Add SMB_EN_MODE property

Add POWER_SUPPLY_PROP_SMB_EN_MODE to indicate the secondary
charger module currently selected for charging via SMB_EN
pin - Charge Pump, SMB1355, or neither.

Also add enum for a list of charger config combos.

Change-Id: I17a489f2103e0ef3045f139ba52de16f819ebb17
Signed-off-by: Harry Yang <harryy@codeaurora.org>