power: qpnp-fg-gen4: Add DT support for Ki current thresh coefficients

Add support for device tree properties to configure low-to-medium and
medium-to-high current thresholds for charging and discharging states.
Also group related SRAM writes into a separate function to reduce
cyclomatic complexity of fg_gen4_hw_init().

Change-Id: If837855f588e5ef5e2268de9e74468cf42a45a55
Signed-off-by: Guru Das Srinagesh <gurus@codeaurora.org>