mtd: spi-nand: Expose spinand_write_reg_op()

This helper function will soon be used from a vendor driver, let's
export it through the spinand.h header. No need for any export, as there
is currently no reason for any module to need it.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20240826101412.20644-6-miquel.raynal@bootlin.com