sdram: fix refresh rate at lower clock speed

Need more frequent refresh when running at lower speed.

fpga/max80.sv

@@ -355,7 +355,7 @@ module max80
 	  .rst_n    ( rst_n ),
 	  .clk      ( sdram_clk ),       // Internal memory clock
 	  .init_tmr ( sys_clk_stb[14] ), // > 100 μs (tP) after reset
-	  .rfsh_tmr ( sys_clk_stb[8] ),  // < 3.9 μs (tREFI/2)
+	  .rfsh_tmr ( sys_clk_stb[6] ),  // < 3.9 μs (tREFI/2)
 
 	  .sr_cs_n  ( sr_cs_n ),
 	  .sr_ras_n ( sr_ras_n ),