Fix boards broken after removal of legacy NAND and DoC support

Commit 2419169f removed support for legacy NAND and disk on chip but
missed to update the code for a few boards. This patch fixes the
resulting build issues.

Signed-off-by: Wolfgang Denk <wd@denx.de>
diff --git a/board/rbc823/rbc823.c b/board/rbc823/rbc823.c
index b294906..e10d9f9 100644
--- a/board/rbc823/rbc823.c
+++ b/board/rbc823/rbc823.c
@@ -256,6 +256,7 @@
 	return (get_ram_size (base, maxsize));
 }
 
+#ifdef CONFIG_CMD_DOC
 void doc_init (void)
 {
 	volatile immap_t *immap = (immap_t *) CONFIG_SYS_IMMR;
@@ -267,3 +268,4 @@
 
 	doc_probe (FLASH_BASE1_PRELIM);
 }
+#endif