spi/davinci: add DT binding documentation

Add binding documentation for spi-davinci module.

[prakash.pm@ti.com: Follow DT naming convention for compatible property]
Signed-off-by: Murali Karicheri <m-karicheri2@ti.com>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Manjunathappa, Prakash <prakash.pm@ti.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>