mxssb: Make use of the CPU enum

Use enum of list of CPU types to improve the typing of the variable.
Signed-off-by: Marek Vasut <marex@denx.de>

mxssb.c

@@ -26,7 +26,8 @@
 
 #include "mxssb.h"
 
-enum {
+enum sb_target_cpu {
+	CPU_UNKNOWN,
 	CPU_MX23,
 	CPU_MX28,
 };
@@ -64,7 +65,7 @@ struct sb_section_ctx {
 };
 
 struct sb_image_ctx {
-	int				target_cpu;
+	enum sb_target_cpu		target_cpu;
 	int				verbose_boot;
 	struct sb_section_ctx		*sect;
 
@@ -960,7 +961,7 @@ int main(int argc, char **argv)
 
 	struct sb_image_ctx ctx;
 	memset(&ctx, 0, sizeof(ctx));
-	ctx.target_cpu = -1;
+	ctx.target_cpu = CPU_UNKNOWN;
 
 	while ((opt = getopt(argc, argv, "c:s:u:o:vh")) != -1) {
 		switch (opt) {