power_supply: Add RESISTANCE property to power supply

Add a "RESISTANCE" property to the power supply frame work that
signifies the internal resistance of the power supply. The units
for this property should be in micro-ohms.

Change-Id: I2fcd3e7e314e61812d364dc0bd85ef1c8eec1622
Signed-off-by: Xiaozhe Shi <xiaozhes@codeaurora.org>
Signed-off-by: David Collins <collinsd@codeaurora.org>