pmic: Convert pm8916 driver to a generic Qcom PMIC driver

Since both pm8916.c and pm8916_gpio.c are already supporting multiple
Qcom SoCs, it makes sense to rename these drivers to pmic_qcom.c and
qcom_pmic_gpio.c respectively. Also, these driver can be extended to
support additional functionality if required for other Qcom SoCs.

Along with this import latest DT binding: qcom,spmi-pmic.txt from Linux
kernel and thereby remove pm8916.txt.

Signed-off-by: Sumit Garg <sumit.garg@linaro.org>