mtd: extend physmap_of to let the device tree specify the parition probe

This is to support custom partitioning schemes for embedded PPC. To use
define your own mtd_part_parser and then add something like:
   linux,part-probe = "my_probe", "cmdlinepart";
To the board's dts file.

If linux,part-probe is not specified then this behaves the same as before.

Signed-off-by: Jason Gunthorpe <jgunthorpe@obsidianresearch.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>