arm: socfpga: stratix10: SPI clock support

This patch is to add SPI clock support for stratix10. Get clock rate
function always returning 0 because the DW-SPI driver get the rate
from clock node in dts but Stratix10 does not support device tree
clock node.To overcome this spi will get the clock_rate directly
from spi clock controller override the weaker function.

Signed-off-by: Dinesh Maniyam <dinesh.maniyam@intel.com>
Reviewed-by: Tien Fong Chee <tien.fong.chee@intel.com>