MIPS: Alchemy: Drop MIRAGE/BOSPORUS board support

No test hardware and no (apparent) users.  These boards seem very
similar to the DB1500, so if required support can be brought back
again (I have datasheets) but then with dedicated board code, not
tacked on to DB1000 support.

Signed-off-by: Manuel Lauss <manuel.lauss@googlemail.com>
To: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/2864/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>