mtd: parser_imagetag: add of_match_table support

Allow matching the imagetag parser for fixed partitions defined in the
device tree.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Richard Weinberger <richard@nod.at>