This will allow it to be built for microwatt out of tree

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

litex/soc/software/bios/sdram.c

@@ -42,6 +42,8 @@ __attribute__((unused)) static void cdelay(int i)
 		__asm__ volatile("nop");
 #elif defined (__rocket__)
 		__asm__ volatile("nop");
+#elif defined (__powerpc__)
+		__asm__ volatile("nop");
 #else
 #error Unsupported architecture
 #endif