fix(imx8m): update poweroff related SNVS_LPCR bits only

Function imx_system_off() writes SNVS_LPCR register to power off the SoC
without bit masking.  This clears other bits like LPWUI_EN and breaks
the function of SoC wake-up using RTC alarm.  Fix it by updating poweroff
related bits only.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Change-Id: If641af4dc1103c67e1a645c03bb36a5f56665aef