arm: mach-imx: imx8m: re-use SNVS init routine

arm: mach-imx: imx8m: re-use SNVS init routine

Working with HAB on the i.MX8MP we've encountered a case where a board
that successfully authenticates u-boot when booting Linux subsequently
fails to properly bring up the RTC.

The RTC registers live in the low-power block of the Secure Non-Volatile
Storage (SNVS) block.

The root cause of the error has been traced to the HAB handing off the
SNVS-RTC in a state where HPCOMR::NPSWA_EN = 0 in other words where the
Non-Privileged Software Access Enable bit is zero.

Configure SNVS to allow unpriv access to SNVS LP for imx8m and imx8mp.

This commit generalizes 723f8359c1 ("imx: mx7: snvs: Add an SNVS init
routine") to also be used on i.MX8M SoCs, and was testeed on i.MX8MP.

Signed-off-by: Ian Ray <ian.ray@gehealthcare.com>