dt-bindings: mtd: nand: Standardize the child node name

In almost all the schema mentioning a NAND chip child node, the name of
the subnode contains a single index number.

In practice there are currently no controller supporting more than 8 cs
so even the [a-f] numbers are not needed. But let's be safe and limit
the number of touched files by just allow a single number everywhere, so
in practice up to 16 CS at most. This value can anyway be limited in
each schema.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/linux-mtd/20221114090315.848208-6-miquel.raynal@bootlin.com