pcmcia: at91_cf: depend on !ARCH_MULTIPLATFORM

Until the driver is corrected to stop including mach/at91_ramc.h and using
mach/io.h, it won't compile in a ARCH_MULTIPLATFORM configuration.

Suggested-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Olof Johansson <olof@lixom.net>