spi/omap: Add DT support to McSPI driver

Add device tree support to the OMAP2+ McSPI driver.
Add the bindings documentation.

Based on original code from Rajendra.

Signed-off-by: Benoit Cousson <b-cousson@ti.com>
Cc: Grant Likely <grant.likely@secretlab.ca>
Cc: Rajendra Nayak <rnayak@ti.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>