spi: cadence_qspi: Migrate CONFIG_CQSPI_REF_CLK to Kconfig

This is a little tricky since SoCFPGA has code to determine this as
runtime.  Introduce a guard variable for platforms to select if they
have a static value to use.  Then for ARCH_SOCFPGA, call
cm_get_qspi_controller_clk_hz() and otherwise continue the previous
behavior.

Cc: Jagan Teki <jagan@amarulasolutions.com>
Signed-off-by: Tom Rini <trini@konsulko.com>