mtd: concat: implement _is_locked mtd operation

Add an implementation of the _is_locked operation for concatenated mtd
devices. This doesn't handle getting the lock status of a range that
spans chips, which is consistent with cfi_ppb_is_locked and
cfi_intelext_is_locked which only look at the first block in the range.

Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Signed-off-by: Richard Weinberger <richard@nod.at>