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/ramtron.c b/drivers/mtd/spi/ramtron.c
index d50da37..a23032c 100644
--- a/drivers/mtd/spi/ramtron.c
+++ b/drivers/mtd/spi/ramtron.c
@@ -35,6 +35,7 @@
 
 #include <common.h>
 #include <malloc.h>
+#include <spi.h>
 #include <spi_flash.h>
 #include "sf_internal.h"