mtd: mchp23k256: add partitioning support

Setting the of_node for the mtd device allows the generic mtd code to
setup the partitions.

[Editorial note (Brian): patch still pending on fixing up the "aligned
to eraseblock" partition sanity check, given that this SRAM has no
eraseblocks.]

Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Tested-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>