Update pmic_bias_tool percentage bias to be relative to nominal

Will use attribute value as the base (nominal) value. Will check that
the value is non-zero before using it, in case the attributes are
not set (for example during pmic_enable manual mode)

Change-Id: I89db022cd7867ce8986cc4a5a36c52f8fde9a355
Reviewed-on: http://rchgit01.rchland.ibm.com/gerrit1/92393
Tested-by: FSP CI Jenkins <fsp-CI-jenkins+hostboot@us.ibm.com>
Tested-by: Jenkins Server <pfd-jenkins+hostboot@us.ibm.com>
Tested-by: Hostboot CI <hostboot-ci+hostboot@us.ibm.com>
Reviewed-by: Louis Stermole <stermole@us.ibm.com>
Reviewed-by: STEPHEN GLANCY <sglancy@us.ibm.com>
Reviewed-by: Jennifer A Stofer <stofer@us.ibm.com>
Reviewed-on: http://rchgit01.rchland.ibm.com/gerrit1/92419
Tested-by: Jenkins OP Build CI <op-jenkins+hostboot@us.ibm.com>
Tested-by: Jenkins OP HW <op-hw-jenkins+hostboot@us.ibm.com>
Reviewed-by: Daniel M Crowell <dcrowell@us.ibm.com>