Add a comment notifying that "device axp" requires miibus for build.

Although if RJ-45 interface is not being used the miibus is not required
but miibus is a build time dependency.

Reviewed by: imp, manu, rajesh1.kumar@amd.com
Approved by: imp, manu, rajesh1.kumar@amd.com
Differential Revision: https://reviews.freebsd.org/D28465