spi: Move SPI related things into new chipdrivers/spi.h

A few things that rely heavily on `flash.h` are moved there instead:
* function signatures containing `erasefunc_t`,
* the inline default_wrsr_target() that needs to know struct flashctx.

This allows to keep the new header file free of a transitive `flash.h`
include.

Change-Id: Ib215821feeb822ea3fc11bf9f48c0328f9a394d4
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.sourcearcade.org/c/flashprog/+/416
diff --git a/spi95.c b/spi95.c
index bd75b17..b7a7b21 100644
--- a/spi95.c
+++ b/spi95.c
@@ -19,8 +19,10 @@
  */
 #include <string.h>
 #include <stdlib.h>
+
+#include "flash.h"
 #include "flashchips.h"
-#include "chipdrivers.h"
+#include "chipdrivers/spi.h"
 #include "spi_command.h"
 #include "spi.h"