mfd: qcom-spmi-pmic: Add remove API

Add remove API for unloading spmi-pmic module as the SPMI
framework right now requires drivers under it to have a
remove API defined when removing them.

Change-Id: Iaf5b9f602fb9389c75a68702eaceb847abcf2876
Signed-off-by: Jishnu Prakash <quic_jprakash@quicinc.com>