mtd: spi-nor: move function declaration out of sfdp.h

sfdp.h should only contain constants related to the JEDEC SFDP
specification(s).

Signed-off-by: Michael Walle <michael@walle.cc>
Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Reviewed-by: Takahiro Kuwano <Takahiro.Kuwano@infineon.com>
Link: https://lore.kernel.org/r/20220810220654.1297699-5-michael@walle.cc