spi: pxa2xx-pci: fix ACPI-based enumeration of SPI devices

Slave devices are not enumerated by ACPI data because the ACPI handle for the
core driver is NULL if it was enumerated by PCI.

Propagate firmware node handle of the PCI device to the platform device.

Suggested-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>