spi: pxa2xx: Remove CONFIG_SPI_PXA2XX_DMA

After removal of legacy PXA DMA code by the commit 6356437e
("spi: spi-pxa2xx: remove legacy PXA DMA bits") the
CONFIG_SPI_PXA2XX_DMA follows the CONFIG_SPI_PXA2XX and cannot be disabled
alone. Therefore remove this config symbol and dead definitions from the
spi-pxa2xx.h.

Signed-off-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>