mmc: sdhci-of-at91: Implement specific ->set_clock() function

Disabling the internal clock while configuring the SD card clock can
lead to internal clock stabilization issue and/or unexpected switch to
the base clock when using presets.
A quirk SDHCI_QUIRK2_NEED_DELAY_AFTER_INT_CLK_RST was introduced to fix
these bugs. The cause was assumed to be a too long internal
re-synchronisation but it seems in some cases the delay (even if longer)
doesn't fix this bug. The safest workaround is to not disable/enable the
internal clock during the SD card clock configuration.

Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>