mtd: nand: sunxi: Convert from fdtdec to ofnode

mtd: nand: sunxi: Convert from fdtdec to ofnode

As a first step toward converting this driver to the driver model, use
the ofnode abstraction to replace direct references to the FDT blob.

Using ofnode_read_u32_index removes an extra pair of loops and makes the
allwinner,rb property optional, matching the devicetree binding.

Signed-off-by: Samuel Holland <samuel@sholland.org>
Acked-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>