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/pcippc2/pcippc2.c b/board/pcippc2/pcippc2.c
index 7985f7d..6ee44d6 100644
--- a/board/pcippc2/pcippc2.c
+++ b/board/pcippc2/pcippc2.c
@@ -147,10 +147,12 @@
 	cpc710_pci_enable_timeout ();
 }
 
+#ifdef CONFIG_CMD_DOC
 void doc_init (void)
 {
 	doc_probe (pcippc2_fpga1_phys + HW_FPGA1_DOC);
 }
+#endif
 
 void pcippc2_cpci3264_init (void)
 {