power: qpnp-fg: add support for holding soc at 100 when full

Add support for the optional property qcom,hold-soc-while-full. When
this property is defined, the fuel gauge driver will report 100% SoC
whenever the battery is full.

CRs-Fixed: 864029
Change-Id: Ide23402f04e1ed71c4d8d9839522e4198b222e35
Signed-off-by: Xiaozhe Shi <xiaozhes@codeaurora.org>