mmc: sdhci-dove: kill the driver specific private struct

There is only one "clk" member in this driver specific private struct.
Actually we can use the "clk" member in the struct sdhci_pltfm_host,
and then kill this struct completely.

Signed-off-by: Kevin Hao <haokexin@gmail.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>