mmc: sdhci-esdhc: move common esdhc_set_clock to platform driver

We need a lot of imx6 specific things into common esdhc_set_clock
for support SD3.0 and eMMC DDR mode which is not needed for power pc
platforms, so esdhc_set_clock seems not so common anymore.

Instead of keeping add platform specfics things into this common API,
we choose to move that code into platform driver itself to handle.
This can also exclude the dependency between imx and power pc on this
headfile and is easy for maintain in the future.

Signed-off-by: Dong Aisheng <b29396@freescale.com>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Chris Ball <cjb@laptop.org>