modules: openthread: radio: encapsulate OT 32-bit timestamp

OT does not have 64 bit timestamp support. This is a limitation of OT
and not of the IEEE 802.15.4 driver API. Therefore any workaround
related to such OT idiosyncracies should be encapsulated inside the OT
adapatation layer.

This change moves the OT-specific conversion of OT 32 bit timestamps to
Zephyr 64 bit timestamps into the OT adaptation layer.

Signed-off-by: Florian Grandel <fgrandel@code-for-humans.de>