dt-bindings: qpnp-fg-gen4: Add ki current threshold properties

Add device tree properties to configure low-to-medium and medium-to-high
current thresholds for charging and discharging.

Change-Id: Iad8757b3afebb99dab1530cd69e5e76200ee17f6
Signed-off-by: Guru Das Srinagesh <gurus@codeaurora.org>