sf: Tidy up public and private header files

Since spi_flash.h is supposed to be the public API for SPI flash, move
private things to sf_internal.h. Also tidy up a few comment nits.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Jagannadha Sutradharudu Teki <jagannadh.teki@gmail.com>
diff --git a/drivers/mtd/spi/spi_spl_load.c b/drivers/mtd/spi/spi_spl_load.c
index 59cca0f..2e0c871 100644
--- a/drivers/mtd/spi/spi_spl_load.c
+++ b/drivers/mtd/spi/spi_spl_load.c
@@ -10,6 +10,7 @@
  */
 
 #include <common.h>
+#include <spi.h>
 #include <spi_flash.h>
 #include <spl.h>