dt-bindings: mtd: qcom_nandc: document qcom,boot-partitions binding

Document new qcom,boot-partition binding used to apply special
read/write layout to boot partitions.

QCOM apply a special layout where spare data is not protected
by ECC for some special pages (used for boot partition). Add
Documentation on how to declare these special pages.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Manivannan Sadhasivam <mani@kernel.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20220616001835.24393-4-ansuelsmth@gmail.com