net: stmmac: rename devlink parameter ts_coarse into phc_coarse_adj

net: stmmac: rename devlink parameter ts_coarse into phc_coarse_adj

The devlink param "ts_coarse" doesn't indicate that we get coarse
timestamps, but rather that the PHC clock adjusments are coarse as the
frequency won't be continuously adjusted. Adjust the devlink parameter
name to reflect that.

The Coarse terminlogy comes from the dwmac register naming, update the
documentation to better explain what the parameter is about.

With this change, the parameter can now be adjusted using:
  devlink dev param set <dev> name phc_coarse_adj value true cmode runtime

Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Link: https://patch.msgid.link/20251030182454.182406-1-maxime.chevallier@bootlin.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>