lib: utils/serial: Use fdt_driver for initialization

The serial driver subsystem does not need any extra data, so it can use
`struct fdt_driver` directly. The generic fdt_serial_init() first
attempts to match the chosen stdout device, and upon failure matches the
first available serial device in the DT. It is a fatal error if no such
device is found. This matches the behavior of fdt_driver_init_one().

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

lib/utils/serial/fdt_serial_htif.c

@@ -41,7 +41,7 @@ static int serial_htif_init(const void *fdt, int nodeoff,
 	return htif_serial_init(custom, fromhost_addr, tohost_addr);
 }
 
-struct fdt_serial fdt_serial_htif = {
+const struct fdt_driver fdt_serial_htif = {
 	.match_table = serial_htif_match,
 	.init = serial_htif_init
 };