Merge branch 'add-100-base-x-mode'

Bjarni Jonasson says:

====================
Add 100 base-x mode

Adding support for 100 base-x in phylink.
The Sparx5 switch supports 100 base-x pcs (IEEE 802.3 Clause 24) 4b5b encoded.
These patches adds phylink support for that mode.

Tested in Sparx5, using sfp modules:
Axcen 100fx AXFE-1314-0521 (base-fx)
Axcen 100lx AXFE-1314-0551 (base-lx)
HP SFP 100FX J9054C (bx-10)
Excom SFP-SX-M1002 (base-lx)

v1 -> v2:
  Added description to Documentation/networking/phy.rst
  Moved PHY_INTERFACE_MODE_100BASEX to above 1000BASEX
  Patching against net-next
====================

Link: https://lore.kernel.org/r/20210113115626.17381-1-bjarni.jonasson@microchip.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>