mmc: sdhci: set the .remove to sdhci_pltfm_unregister()

In these drivers, the driver specific .remove function just a simple
wrapper of function sdhci_pltfm_unregister(). So remove these wrappers
and just set .remove to sdhci_pltfm_unregister().

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