leds: qpnp-flash-v2: add support to query battery parameters

Register a notifier callback with the power_supply framework
to detect when the bms power_supply is available so that
it can be used to query the battery properties required from
the fuel gauge for the maximum current query algorithm.

CRs-Fixed: 1043718
Change-Id: Icd535025a2f888e188f89841f32ac5b822f9b20d
Signed-off-by: Devesh Jhunjhunwala <deveshj@codeaurora.org>