spi/pxa2xx: embed the ssp_device to platform data

The spi-pxa2xx-pci glue driver had to implement pxa_ssp_request()/free() in
order to support the spi-pxa2xx platform driver. Since the ACPI enabled
platforms can use the same platform driver we would need to implement
pxa_ssp_request()/free() in some central place that can be shared by the
ACPI and PCI glue code.

Instead of doing that we can make pxa_ssp_request()/free() to be available
only when CONFIG_ARCH_PXA is set. On other arches these are being stubbed
out in preference to passing the ssp_device from the platform data
directly.

We also change the SPI bus number to be taken from ssp->port_id instead of
platform device id. This way the supporting code that passes the ssp can
decide the number (or it can set it to the same as pdev->id).

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>