ARM: dts: imx6: drop gpmi-nand address and size cells

Since commit fe2585e9 ("doc: dt: mtd: support partitions in a
special 'partitions' subnode") and commit 5cfdedb7

 ("mtd: ofpart:
move ofpart partitions to a dedicated dt node") the partitioning should
be within a partitions sub-node.

Baseboard device trees following that scheme will get a dtc warning due
to the predefined #address-cells and #size-cells properties:

arch/arm/boot/dts/imx6qdl.dtsi:171.26-189.5: Warning (avoid_unnecessary_addr_size): /soc/gpmi-nand@112000: unnecessary #address-cells/#size-cells without "ranges" or child "reg" property

I found no upstream imx6 baseboard using the old partitioning scheme, so
we can drop the two properties to avoid such warnings.

Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>