util/sconfig: Get rid of struct header

Now that chips and devices are treated differently in sconfig, this
change gets rid of struct header and add_header function which were
responsible for maintaining list of headers that need to be added to
static.c.

Instead, struct chip is re-factored into struct chip and
struct chip_instance, where chip is a list of unique chips required by
the mainboard whereas chip_instance is an instance of the chip. One
chip can have multiple instances dependending upon the devices in the
system. Also, struct device is updated to hold a pointer to chip
instance instead of the chip structure. This unique list of chips is
then used to add appropriate headers to static.c

BUG=b:80081934
TEST=Verified using abuild that all boards compile successfully.

Change-Id: I6fccdf7c361b4f55a831195adcda9b21932755aa
Signed-off-by: Furquan Shaikh <furquan@google.com>
Reviewed-on: https://review.coreboot.org/26739
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

util/sconfig/sconfig.tab.h_shipped

@@ -86,7 +86,7 @@ union YYSTYPE
 
 
 	struct device *device;
-	struct chip *chip;
+	struct chip_instance *chip_instance;
 	char *string;
 	int number;