mfd: qcom-spmi-pmic: Add support for PM/PMI8950

Add the subtype and compatible strings for PM8950 and PMI8950,
found in various SoCs, including MSM8953, MSM8956, MSM8976 and
APQ variants.

Signed-off-by: Angelo G. Del Regno <kholk11@gmail.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>