Mellanox mlx5_core_en not set for x86_64
It appears that CONFIG_MLX5_CORE_EN=y for aarch64 but unset for x86_64. This config option CONFIG_MLX5_CORE_EN=y seems to be required in order to create NICs under /sys/class/net/ and without it, newer Mellanox NICs are not usable under this architecture.
https://git.alpinelinux.org/cgit/aports/tree/main/linux-vanilla/config-vanilla.aarch64?h=3.5-stable
https://git.alpinelinux.org/cgit/aports/tree/main/linux-vanilla/config-vanilla.x86\_64?h=3.5-stable
This option appear to be unset from 3.x all the way to master. This Mellanox hardware is most likely to be available on x86_64 and NOT aarch64 (where it is currently set/enabled).
(from redmine: issue id 7410, created on 2017-06-09)