parallel.h: Extract parallel declarations to a separate header

This patch moves all the declarations relevant to parallel into their own
header in include/parallel.h
The corresponding functions implementations are already in parallel.c,
so the declarations naturally can be in parallel.h
Currently, most of the declarations reside in flash.h making it difficult
to really understand file dependency.

Change-Id: I69f6e224a7ece373fbd6606e802930a52da1dd85
Signed-off-by: Antonio Vázquez <antoniovazquezblanco@gmail.com>
Reviewed-on: https://review.coreboot.org/c/flashrom/+/89503
Reviewed-by: Anastasia Klimchuk <aklm@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

82802ab.c

@@ -24,6 +24,7 @@
 
 #include <stdbool.h>
 #include "flash.h"
+#include "parallel.h"
 #include "chipdrivers.h"
 
 void print_status_82802ab(uint8_t status)