liblitedram/sdram: Add option to disable cdelay()

When running in sim, those delays can take a *long* time, which
isn't always necessary with the simulated litedram PHY.

This allows system.h to optionally set CONFIG_SIM_DISABLE_DELAYS
which  causes cdelay to do nothing.

This is especially useful when using a verilated litedram inside
a bigger/slower simulated design as to not spend a huge amount
of time going through the initializations.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

litex/soc/software/liblitedram/sdram.c

@@ -29,10 +29,12 @@
 
 __attribute__((unused)) static void cdelay(int i)
 {
+#ifndef CONFIG_SIM_DISABLE_DELAYS
 	while(i > 0) {
 		__asm__ volatile(CONFIG_CPU_NOP);
 		i--;
 	}
+#endif
 }
 
 #ifdef CSR_SDRAM_BASE