mfd: pm8921: Add support for pm8018

In order to support the Qualcomm MDM9615 PMIC, add support for the
pm8018 in pm8921 MFD driver.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>