mmc: sdhci: enable/disable the clock in sdhci_pltfm_suspend/resume

This commit provides similar cleanups as commit 83eacdfa ("mmc:
sdhci: disable the clock in sdhci_pltfm_unregister()") did for
unregister hooks.

sdhci-brcmstb.c and sdhci-sirf.c implement their own suspend/resume
hooks to handle pltfm_host->clk.  Move clock handling to sdhci_pltfm.c
so that the drivers can reuse sdhci_pltfm_pmops.

The following drivers did not previously touch pltfm_host->clk during
suspend/resume, but now do:
  - sdhci-bcm-kona.c
  - sdhci-dove.c
  - sdhci-iproc.c
  - sdhci-pxav2.c
  - sdhci-tegra.c
  - sdhci-xenon.c

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Acked-by: Al Cooper <alcooperx@gmail.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>