In def47c5095d53814512bb0c62ec02dfdec769db1 the AX88796 driver got
restricted to just be build for ARM and MIPS on the sole merrit that it
was written for some ARM sytems and the driver had the missfortune to
just build on MIPS, so MIPS was throw into the dependency for a good
measure. Later 8687991a734a67f1638782c968f46fff0f94bb1f added SH but
only one in-tree SH system actually has an AX88796.
Tighten up dependencies by using an auxilliary config sysmbol
HAS_NET_AX88796 which is selected only by the platforms that actually
have or may have an AX88796. This also means the driver won't be built
anymore for any MIPS platform.
Signed-off-by: Ralf Baechle<ralf@xxxxxxxxxxxxxx>