NAND boot: MPC8313ERDB support

Note that with older board revisions, NAND boot may only work after a
power-on reset, and not after a warm reset.  I don't have a newer board
to test on; if you have a board with a 33MHz crystal, please let me know
if it works after a warm reset.

Signed-off-by: Scott Wood <scottwood@freescale.com>
diff --git a/board/freescale/mpc8313erdb/sdram.c b/board/freescale/mpc8313erdb/sdram.c
index afd8b9d..3a6347f 100644
--- a/board/freescale/mpc8313erdb/sdram.c
+++ b/board/freescale/mpc8313erdb/sdram.c
@@ -58,8 +58,10 @@
  */
 static long fixed_sdram(void)
 {
-	volatile immap_t *im = (volatile immap_t *)CFG_IMMR;
 	u32 msize = CFG_DDR_SIZE * 1024 * 1024;
+
+#ifndef CFG_RAMBOOT
+	volatile immap_t *im = (volatile immap_t *)CFG_IMMR;
 	u32 msize_log2 = __ilog2(msize);
 
 	im->sysconf.ddrlaw[0].bar = CFG_DDR_SDRAM_BASE >> 12;
@@ -100,6 +102,7 @@
 
 	/* enable DDR controller */
 	im->ddr.sdram_cfg |= SDRAM_CFG_MEM_EN;
+#endif
 
 	return msize;
 }