SOCFPGA: clock manager: implement dw_spi_get_clk function

Implement dw_spi_get_clk function to override its weak
implementation in designware_spi.c driver.

We need this change to get rid of cm_get_spi_controller_clk_hz
function and clock_manager.h include in designware_spi.c driver.

Reviewed-by: Marek Vasut <marex@denx.de>
Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>