generic: backport some phylink helper functions

generic: backport some phylink helper functions

It isn't feasible to literally backport all upstream phylink_pcs changes
down to Linux 5.15: It's just too many patches, and many downstream
drivers and hacks are likely to break. We are too close to branching off
to risk this, and it's also just too much work.
Instead just add helper functions used by modern PCS drivers while keeping
the original functions instact as well. While this may add a kilobyte or
two of extra kernel size, it has the advantage that we get the best of both
worlds: None of the existing codepaths are touched, but yet we have the
option to backport singular improvements to Ethernet drivers where needed.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>