mtd: denali: split the generic driver and PCI layer

The Denali controller can also be found in SoC devices attached to a
simple bus.  Move the PCI specific parts into denali_pci so that we can
add a denali_dt that uses the same driver but for a device tree driver
instead of a PCI based device.

Signed-off-by: Jamie Iles <jamie@jamieiles.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>