ARM: pxa: ssp: add DT bindings

This patch contains an ugly hack for looking up the the DMA request
number. The problem here is that the implementation as it stands will
allocate the DMA channel from the user of the ssp port, and hence we
cannot allocate a real channel here.

Signed-off-by: Daniel Mack <zonque@gmail.com>
Acked-by: Haojian Zhuang <haojian.zhuang@gmail.com>
Signed-off-by: Mark Brown <broonie@linaro.org>