net: gptp: Convert SyncReceiptTime to ExtendedTimestamp

SyncReceiptTime should use an ExtendedTimestamp (with fractional
nanoseconds precision). Add a struct with the definition of the needed
type and convert that variable.

The struct representing the ExtendedTimestamp is named
net_ptp_extended_time to keep consistency with the existing net_ptp_time
which is used for regular PTP timestamps.

See 802.1AS-2011 chapters 10.2.3.4 and 6.3.3.5 for more reference.

Signed-off-by: Tomasz Gorochowik <tgorochowik@antmicro.com>