mtd: fix section mismatch for doc_probe_device

doc_probe_device() is only called from docg3_probe() which is in .init.text,
so it must be in the same section to avoid a section mismatch warning.

Signed-off-by: Ryosuke Saito <raitosyo@gmail.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>