mmc: Add support for wait_dat0 callback

There is no wait_dat0 mmc ops, causing operations waiting for data
line state change (e.g mmc_switch_voltage) to fallback to a 250ms
active delay. mmc_ops still used when DM_MMC is not enabled, which
is often the case for SPL. The result can be unexpectly long SPL
boot time.

This change adds support for wait_dat0() mmc operation.

Signed-off-by: Loic Poulain <loic.poulain@linaro.org>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>