Integrate u-boot-seco-imx/seco_lf_v2021.04-gm_d18_rtc

Commit: edgehog/bsp/nxp/u-boot-seco-imx@67f8d7f9

[D18] setup RTC PMIC voltage

Raise RTC PMIC voltage from 1.8V a 1.85V to better stay within spec.