power: qpnp-fg: support variable sram update periods

Add support to modify the fuel gauge sram refresh rate via the module
parameter sram_update_period_ms.

Change-Id: I94dab77d15c66f944a35ec644a95d746a68f3d43
Signed-off-by: Xiaozhe Shi <xiaozhes@codeaurora.org>
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>