spi: add ti-ssp spi master driver

This patch adds an SPI master implementation that operates on top of an
underlying TI-SSP port.

Acked-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Cyril Chemparathy <cyril@ti.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>
Signed-off-by: Kevin Hilman <khilman@ti.com>