power_supply: Add boot and calibration attributes

Usually PMIC's come with coulomb counting mechanism which can be
used to implement a Fuel Gauginig solution in Software itself.
One of key input to these SW Fuel Gauge solutioons is the boot up
parameters like boot voltage and boot current.

This patch adds the VOLTAGE_BOOT and CURRENT_BOOT power supply attributes
to report bootup voltage and current.

This patch also adds CALIBRATE power supply attribute which useful is
for calibrating the battery/coulomb counter.

Signed-off-by: Ramakrishna Pallala <ramakrishna.pallala@intel.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>