mtd: spi-nor-core: Move SFDP related declarations to top

These structures will be used in a later commit inside another structure
definition. Also take the declarations out of the ifdef since they won't
affect the final binary anyway and will be used in a later commit.

Signed-off-by: Pratyush Yadav <p.yadav@ti.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>