ARM: pxa: ssp: add pxa_ssp_request_of()

Add a function to lookup ssp devices from device tree. This way, users
can reference the ssp devices in order to register to them.

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