ARM: footbridge: don't build floppy code for addin mode

The ARCH_EBSA285_ADDIN platform does not provide the
ISA_DMA API, which is required by the floppy driver.

Let's ensure that the floppy code can only be built
when ISA_DMA is also enabled, by moving the select
statement into ARCH_EBSA285_HOST.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Russell King <linux@arm.linux.org.uk>