bcache: remove EXPERIMENTAL for Kconfig option 'Asynchronous device registration'

The "Asynchronous device registration (EXPERIMENTAL)" Kconfig option is
for 2+ years, it is used when registration takes too much time for
massive amount of cached data, to avoid udev task timeout during boot

Many users and products enable this Kconfig option for quite long time
(e.g. SUSE Linux) and it works as expected and no issue reported.

It is time to remove the "EXPERIMENTAL" tag from this Kconfig item.

Signed-off-by: Coly Li <colyli@suse.de>
diff --git a/drivers/md/bcache/Kconfig b/drivers/md/bcache/Kconfig
index cf3e809..529c9d0 100644
--- a/drivers/md/bcache/Kconfig
+++ b/drivers/md/bcache/Kconfig
@@ -29,7 +29,7 @@
 	operations that get stuck.
-	bool "Asynchronous device registration (EXPERIMENTAL)"
+	bool "Asynchronous device registration"
 	depends on BCACHE
 	Add a sysfs file /sys/fs/bcache/register_async. Writing registering