mtd: partitions: factor out code calling parser

This code is going to be reused for parsers matched using OF so let's
factor it out to make this easier.

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