Merge branch 'net-phy-let-phy_speed_down-up-support-speeds-1Gbps'

Heiner says:

====================
So far phy_speed_down/up can be used up to 1Gbps only. Remove this
restriction and add needed helpers to phy-core.c

v2:
- remove unused parameter in patch 1
- rename __phy_speed_down to phy_speed_down_core in patch 2
====================

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>