mtd: spinand: make spinand_{wait,otp_page_size} global

Change the functions spinand_wait() and spinand_otp_page_size() from
static to global so that SPI NAND flash drivers don't duplicate it.

Signed-off-by: Martin Kurbanov <mmkurbanov@salutedevices.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>