dt-bindings: mtd: intel: lgm-nand: Fix maximum chip select value

The Intel LGM NAND IP only supports two chip selects: There's only two
CS and ADDR_SEL register sets. Fix the maximum allowed chip select value
according to the dt-bindings.

Fixes: 2f9cea8e ("dt-bindings: mtd: Add Nand Flash Controller support for Intel LGM SoC")
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20220702231227.1579176-3-martin.blumenstingl@googlemail.com