microblaze: Add SPL support

Add support for U-BOOT SPL. NOR and RAM mode are supported.
There are 3 images in NOR flash. u-boot.img, dtb and kernel.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
diff --git a/arch/microblaze/cpu/timer.c b/arch/microblaze/cpu/timer.c
index 69ae6d4..3960bbb 100644
--- a/arch/microblaze/cpu/timer.c
+++ b/arch/microblaze/cpu/timer.c
@@ -34,6 +34,7 @@
 	}
 }
 
+#ifndef CONFIG_SPL_BUILD
 static void timer_isr(void *arg)
 {
 	timestamp++;
@@ -62,10 +63,15 @@
 		if (ret)
 			tmr = NULL;
 	}
-
 	/* No problem if timer is not found/initialized */
 	return 0;
 }
+#else
+int timer_init(void)
+{
+	return 0;
+}
+#endif
 
 /*
  * This function is derived from PowerPC code (read timebase as long long).