drivers: ieee802154: nRF5: TX timestamp now refers to start of PHR

Based on the standard based definitions given in previous commits, the
TX timestamp used for timed TX now refers to the start of PHR. As OT
continues to calculate timestamps based on a "start of SHR" definition,
the duration of the PHY specific SHR is added in the OT adaptation layer
to make up for this OT quirk.

Fixes: #59245

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