mtd: spinand: otp: add helpers functions

The global functions spinand_otp_read() and spinand_otp_write() have
been introduced. Since most SPI-NAND flashes read/write OTP in the same
way, let's define global functions to avoid code duplication.

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