power: pm8921-bms: provide option to ignore shutdown soc

Few platforms do not want to use the shutdown soc. Provide a option to
selectively disable it.

Change-Id: Ie57d474f4e81e4ce2e7e7a4a92cda20d4c8dd184
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>