mmc: cavium-octeon: Convert to use module_platform_driver

Get rid of boilerplate code by using module_platform_driver macro.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Jan Glauber <jglauber@cavium.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>