rtc: rtc-pm8xxx: Add support for pm8018 rtc

In order to support RTC on Qualcomm MDM9615 SoC, add support for
the pm8018 rtc in rtc-pm8xxx driver.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Acked-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>