lib: utils/serial: Constify FDT pointers in parsing functions

Indicate that none of these functions modify the devicetree by
constifying the parameter type.

Signed-off-by: Samuel Holland <samuel.holland@sifive.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Anup Patel <anup@brainfault.org>

include/sbi_utils/serial/fdt_serial.h

@@ -16,7 +16,7 @@
 
 struct fdt_serial {
 	const struct fdt_match *match_table;
-	int (*init)(void *fdt, int nodeoff, const struct fdt_match *match);
+	int (*init)(const void *fdt, int nodeoff, const struct fdt_match *match);
 };
 
 int fdt_serial_init(void);