mtd: spi-nor: otp: use more consistent wording

Use the wording as used in the datasheet to describe the access methods
of the security registers (aka OTP storage). This will also match the
function names.

Signed-off-by: Michael Walle <michael@walle.cc>
Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Reviewed-by: Pratyush Yadav <p.yadav@ti.com>
Reviewed-by: Tudor Ambarus <tudor.ambarus@microchip.com>