spi: Add SPI master controller for OCTEON SOCs.

Add the driver, link it into the kbuild system and provide device tree
binding documentation.

Signed-off-by: David Daney <david.daney@cavium.com>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Patchwork: http://patchwork.linux-mips.org/patch/4292/
Signed-off-by: John Crispin <blogic@openwrt.org>