spi: ep93xx: add DT support for Cirrus EP93xx

- add OF ID match table
- add device tree DMA request, so we can probe defer, in case DMA is not
  ready yet
- drop DMA platform code

Signed-off-by: Nikita Shubin <nikita.shubin@maquefel.me>
Tested-by: Alexander Sverdlin <alexander.sverdlin@gmail.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Mark Brown <broonie@kernel.org>
Acked-by: Alexander Sverdlin <alexander.sverdlin@gmail.com>
Acked-by: Vinod Koul <vkoul@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>