pcmcia: db1xxx_ss: restrict to MIPS_DB1XXX boards

When the MIPS_ALCHEMY board selection is MIPS_XXS1500 instead of
MIPS_DB1XXX, the PCMCIA driver 'db1xxx_ss' has build errors due
to missing DB1XXX symbols. The PCMCIA driver should be restricted
to MIPS_DB1XXX instead of MIPS_ALCHEMY to fix this build error.

ERROR: modpost: "bcsr_read" [drivers/pcmcia/db1xxx_ss.ko] undefined!
ERROR: modpost: "bcsr_mod" [drivers/pcmcia/db1xxx_ss.ko] undefined!

Fixes: 42a4f17d ("MIPS: Alchemy: remove SOC_AU1X00 in favor of MIPS_ALCHEMY")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reported-by: kernel test robot <lkp@intel.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Kees Cook <keescook@chromium.org>
Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc: linux-mips@vger.kernel.org
Acked-by: Manuel Lauss <manuel.lauss@gmail.com>
Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>