net: ptp: Add TLV extension to PTP

Add allocation of container used for TLVs and dummy functions for
preparing correct byte order of data. TLV stands for Type, Length,
Value, and it is extension of PTP messages that's used to transmit
extra information. This data is attached at the end of PTP message.

Signed-off-by: Adam Wojasinski <awojasinski@baylibre.com>