greybus: sdio: extend sdio implementation

Extend sdio implementation, as it for now it was basically stubs.

This implementation is compile tested only since there is no fw or
simulation support yet.
Next step is to add sdio support to gbsim and test it with success
using the mmc_test facility.

Signed-off-by: Rui Miguel Silva <rui.silva@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>