iio: common: NVS v209 common timestamps

iio: common: NVS v209 common timestamps

- Create a common timestamp function for NVS sensor drivers.
- Timestamp changed from ktime_get_ts to get_monotonic_boottime to address
  the timestamp change through suspend.

- Removed the dependency of the NVI driver on NVS drivers supporting the NVI
  auxiliary bus.  If NVI_MPU is not enabled then these drivers build without
  NVI auxiliary bus API support.

Bug 200142073

Change-Id: I53b16bfb39b53015f3ee1e5418755f9b3f0cd132
Signed-off-by: Erik Lilliebjerg <elilliebjerg@nvidia.com>
Reviewed-on: http://git-master/r/821141
(cherry picked from commit 7c5801f839a493f17fa81993cc6b2315716c737a)
Reviewed-on: http://git-master/r/823563
Reviewed-by: Automatic_Commit_Validation_User
GVS: Gerrit_Virtual_Submit
Reviewed-by: Robert Collins <rcollins@nvidia.com>
Tested-by: Robert Collins <rcollins@nvidia.com>