mmc: Move MMC boot code into its own file

Rather than having an #ifdef in the main mmc.c file, control this feature
from the Makefile by moving the code into its own file.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/mmc/mmc_private.h b/drivers/mmc/mmc_private.h
index cfc4aca..3d189ba 100644
--- a/drivers/mmc/mmc_private.h
+++ b/drivers/mmc/mmc_private.h
@@ -106,4 +106,15 @@
  */
 int mmc_switch_part(struct mmc *mmc, unsigned int part_num);
 
+/**
+ * mmc_switch() - Issue and MMC switch mode command
+ *
+ * @mmc:	MMC device
+ * @set:	Unused
+ * @index:	Cmdarg index
+ * @value:	Cmdarg value
+ * @return 0 if OK, -ve on error
+ */
+int mmc_switch(struct mmc *mmc, u8 set, u8 index, u8 value);
+
 #endif /* _MMC_PRIVATE_H_ */