mmc: mtk-sd: implement waiting for DAT0 line state

With the recent changes on mmc driver, we saw that the boot is ~5 secs
longer compared to v2021.07 on mediatek platforms.

This regression is seen during mmc_init and caused by the following
patch [1].

Indeed since we did not support poll dat0, we fulfilled the condition
of [1] and a delay of 500 ms was added for every __mmc_switch call.

By adding the support of wait_dat0(), we now don't need to mdelay
during mmc_init anymore.

[1]: https://patchwork.ozlabs.org/project/uboot/patch/[email protected]/

Signed-off-by: Julien Masson <[email protected]>
Reviewed-by: Jaehoon Chung <[email protected]>

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