mtd: partitions: rename "master" to the "parent" where appropriate

This prepares mtd subsystem for the new feature: subpartitions. In some
cases flash device partition can be a container with extra subpartitions
(volumes).

So far there was a flat structure implemented. One master (flash device)
could be partitioned into few partitions. Every partition got its master
and it was enough to get things running.

To support subpartitions we need to store pointer to the parent for each
partition. This is required to implement more natural tree structure and
handle all recursion and offsets calculation.

To make code consistent this patch renamed "master" to the "parent" in
places where we can be dealing with subpartitions.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>