net: openthread: align platform time with radio time

On dual-core architecture the platform time can be not aligned with
radio time. This happens e.g. for nRF53 devices. Unaligned times imply
mulfunction in CSL windows scheduling.
This PR fixes it by adding/subtracting the time offset in functions
which return the plaform time. The changes have no impact on platforms
where the times are the same.

Signed-off-by: Lukasz Maciejonczyk <lukasz.maciejonczyk@nordicsemi.no>