mmc: sh_mobile_sdhi: add new macro for mmc_host to sh_mobile_sdhi

getting struct sh_mobile_sdhi from struct mmc_host needs to use
complex container_of(). This patch adds new host_to_priv macro to
get it easily.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>