mtd: rawnand: nandsim: Add NS_PAGE_BYTE_SHIFT macro to replace the repeat pattern

The (ns->regs.column + ns->regs.off) pattern repeats a lot which
represents the byte shift in next page to access. We can replace it
with a macro to improve the readability.

Signed-off-by: RinHizakura <s921975628@gmail.com>
Reviewed-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20211225100713.119089-1-s921975628@gmail.com