spi/pxa2xx: add support for SPI_LOOP

This is useful when testing the functionality of the controller from userspace
and there aren't any real SPI slave devices connected to the bus.

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