net: gptp: Initial GrandMaster capability support

Implements GMCAP-1, GMCAP-2, and GMCAP-3 and their dependencies from
802.1AS-2011. See Annex A.10 for more details.

The Grand Master Capability can be turned on and off through KConfig.

Note: the correction field in FUP packets is not yet properly
calculated. There is a TODO left in the code, near which some parameters
are zeroed to make the correction field be set to 0. This mimics the
behavior of openAvnu (a Linux gPTP client). For full compliance the
field should be calculated and set properly.

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