lib: utils/regmap: Use fdt_driver for initialization

The regmap driver subsystem does not need any extra data, so it can use
`struct fdt_driver` directly. It always initializes the driver for a
specific DT node.

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

lib/utils/regmap/fdt_regmap_syscon.c

@@ -263,7 +263,7 @@ static const struct fdt_match regmap_syscon_match[] = {
 	{ },
 };
 
-struct fdt_regmap fdt_regmap_syscon = {
+const struct fdt_driver fdt_regmap_syscon = {
 	.match_table = regmap_syscon_match,
 	.init = regmap_syscon_init,
 };