Fix axi-spi node generation

Fix bug introduced by
"Support axi_quad IP from Vivado"
(sha1:2f951614e23880650e2831996350c4bfe45997d9)
because axi_spi could be in setup that S_AXI4_ACLK is defined
but frequency is empty. Check if any frequency is setup, if not
go to the default old case and use S_AXI_ACLK instead.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>