rtc: pm8xxx: drop unused pm8018 compatible

The PM8018 compatible is always used with PM8921 fallback, so PM8018
compatible can be safely removed from device ID table

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://lore.kernel.org/r/20220928-mdm9615-dt-schema-fixes-v5-2-bbb120c6766a@linaro.org


Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>