mmc: sdhci-of-at91: factor out clks and presets setting

The setting of clocks and presets is currently done in probe only but
once deep PM support is added, it'll be needed in the resume function.

Let's create a function for this setting.

Signed-off-by: Quentin Schulz <quentin.schulz@free-electrons.com>
Acked-by: Ludovic Desroches <ludovic.desroches@microchip.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>