mtd: spi-nor: intel-spi: support chips without software sequencer

Some flash controllers don't have a software sequencer. Avoid
configuring the register addresses for it, and double check
everywhere that its not accidentally trying to be used.

Every use of `sregs` is now guarded by a check of `sregs` or
`swseq_reg`. The check might be done in the calling function.

Signed-off-by: Jethro Beekman <jethro@fortanix.com>
Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>