mtd: ofpart: grab device tree node directly from master device node

It seems more logical to use a device node directly associated with the
MTD master device (i.e., mtd->dev.of_node field) rather than requiring
auxiliary partition parser information to be passed in by the driver in
a separate struct.

This patch supports the mtd->dev.of_node field and deprecates the parser
data 'of_node' field

Driver conversions may now follow.

Additional side benefit to assigning mtd->dev.of_node rather than using
parser data: the driver core will automatically create a device -> node
symlink for us.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Reviewed-by: Boris Brezillon <boris.brezillon@free-electrons.com>