Add Full Duplex support to SPI (v2)

This patch adds full duplex support to the spi_sunxi driver.

v2 - Fix compile error when CONFIG_SUN4I_SPI_NDMA is set.

Signed-off-by: Simon Ratcliffe <simon@proatomic.com.au>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>