spi: pxa2xx: Update documentation to point out that it's outdated

Update documentation by pointing out that it's applicable mostly
for a legacy platform. While at it, add couple of points with regard
to ACPI, Device Tree, and automatic DMA enablement.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20210517140351.901-9-andriy.shevchenko@linux.intel.com


Signed-off-by: Mark Brown <broonie@kernel.org>