mmc: socfpga: Add clock framework support

Add support for fetching the clock frequency both using the legacy
method in case clock framework is disabled as well as via the clock
framework if it is enabled. This allows for migration to the clock
framework on platforms which supports it while not breaking legacy
platforms. That said, the legacy method must be removed eventually.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Chin Liang See <chin.liang.see@intel.com>
Cc: Dinh Nguyen <dinguyen@kernel.org>
Cc: Ley Foon Tan <ley.foon.tan@intel.com>