mmc: core: Move some host specific public functions to host.h

Ideally the public mmc header file, core.h, shouldn't contain interfaces
particularly intended to be used by host drivers. Instead those should
remain in the host.h header file. Therefore, let's move a couple functions
from core.h to host.h.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Shawn Lin <shawn.lin@rock-chips.com>