net: Make SMC91X selectable on other MIPS boards

RBTX4939 board has SMC91X chip and there can be other MIPS boards with
that chip.  Make SMC91X selectable on all MIPS board would be better than
enumerating each boards in Kconfig.

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Cc: jeff@garzik.org
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>