power: qpnp-fg: support POWER_SUPPLY_PROP_UPDATE_NOW

Add the update_now property to the fuel gauge bms power supply in order
to allow other drivers and users to force fuel gauge updates regardless
of the last time it read from the hardware.

Change-Id: I10148ddc4597ad2edb06884d3b7652c2953c6c8f
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>