MFC mii: Add opt_platform.h to all miibus drivers

miivar.h includes opt_platform.h. Make sure all the drivers that use the
miibus_if.h interface file have opt_platform.h as well. While some of
these may not, strictly speaking, need it, it's easier to include it
universally for miibus.

Sponsored by: Netflix

(cherry picked from commit b111430e54537a66080df115c3a2950a2922da5a)

Author: Warner Losh <imp@FreeBSD.org>