mtd: spi-nor: intel-spi: Add support for Intel Denverton SPI serial flash controller

Intel Denverton exposes the SPI serial flash controller as a PCI device
instead of being part of the LPC chip as previous generations did.

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Cyrille Pitchen <cyrille.pitchen@free-electrons.com>