drivers: spi: Add SPI controller driver for Octeon

Adds support for SPI controllers found on Octeon II/III and Octeon TX
TX2 SoC platforms.

Signed-off-by: Aaron Williams <awilliams@marvell.com>
Signed-off-by: Suneel Garapati <sgarapati@marvell.com>
Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Cc: Aaron Williams <awilliams@marvell.com>
Cc: Chandrakala Chavva <cchavva@marvell.com>
Cc: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>