bitbang_spi: Move API into its own header file

We're going to extent it for multi-i/o.

Change-Id: Ifead97d7a8f848b82a4d21c557f5d364066d5d6a
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.sourcearcade.org/c/flashprog/+/81
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
diff --git a/bitbang_spi.c b/bitbang_spi.c
index c43fa2f..1c2c178 100644
--- a/bitbang_spi.c
+++ b/bitbang_spi.c
@@ -20,6 +20,7 @@
 #include "flash.h"
 #include "programmer.h"
 #include "spi.h"
+#include "bitbang_spi.h"
 
 struct bitbang_spi_master_data {
 	const struct bitbang_spi_master *mst;