mtd: diskonchip: don't warn about ARM architecture

Enabling the diskonchip drivers on most architectures results
in a pointless warning "#warning Unknown architecture for
DiskOnChip. No default probe locations defined". The driver
can in fact handle the default location already through the
CONFIG_MTD_DOCPROBE_ADDRESS, which gets set on the platforms
that need it, and we get a run-time error if this is not
set correctly.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>