Remove the mlx5 driver from LINT kernel config for 32-bit PPC

The mlx5 driver requires 64-bit atomics, which are not supported on 32-bit
PPC. Exclude that driver from the LINT kernel config.

Submitted by: hps (but I re-worded the comment)
Reported by: rpokala
Reviewed by: jhibbits
Differential Revision: https://reviews.freebsd.org/D15611