mmc: cavium: Add MMC support for Octeon SOCs.

Add platform driver for Octeon SOCs.

Signed-off-by: Steven J. Hill <Steven.Hill@cavium.com>
Signed-off-by: David Daney <david.daney@cavium.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>