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>

lib/utils/serial/fdt_serial_htif.c

@@ -18,7 +18,7 @@ static const struct fdt_match serial_htif_match[] = {
 	{ },
 };
 
-static int serial_htif_init(void *fdt, int nodeoff,
+static int serial_htif_init(const void *fdt, int nodeoff,
 			    const struct fdt_match *match)
 {
 	int rc;