spi: add xtfpga SPI controller driver

This simple SPI master controller is built into xtfpga bitstreams. It
always transfers 16 bit words in SPI mode 0, automatically asserting CS
on transfer start and deasserting on end.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Mark Brown <broonie@linaro.org>