mtd: spinand: make spinand_{read,write}_page global

Change these functions from static to global so that to use them later
in OTP operations. Since reading OTP pages is no different from reading
pages from the main area.

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