mtd: lantiq-flash.c: use mtd_device_parse_register

Replace custom invocations of parse_mtd_partitions and mtd_device_register
with common mtd_device_parse_register call. This would bring: standard
handling of all errors, fallback to default partitions, etc.

Signed-off-by: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>