mtd: spi-nor: Guard SPI_STACKED_PARALLEL with DM_SPI check

mtd: spi-nor: Guard SPI_STACKED_PARALLEL with DM_SPI check

While we want to compile the SPI_STACKED_PARALLEL code everywhere we
can, it must first be guarded with an #if for DM_SPI as not all cases
where we have this code built, such as in SPL, will have the relevant
DM_SPI option enabled.

Fixes: 43423cdc5dc1 ("mtd: spi-nor: Always build SPI_STACKED_PARALLEL code")
Reviewed-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
Signed-off-by: Tom Rini <trini@konsulko.com>