netlink: specs: add the multicast group name to spec

Add the multicast group's name to the YAML spec.
Without it YNL doesn't know how to subscribe to notifications.

Reviewed-by: Donald Hunter <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

Documentation/netlink/specs/ethtool.yaml

···
         attributes:
           - header
           - events
+
+mcast-groups:
+  list:
+    -
+      name: monitor
+      c-define-name: ethtool-mcgrp-monitor-name

include/uapi/linux/ethtool_netlink.h

···
 	ETHTOOL_A_STATS_PHY_MAX = (__ETHTOOL_A_STATS_PHY_CNT - 1)
 };
 
-#define ETHTOOL_MCGRP_MONITOR_NAME "monitor"
-
 #endif /* _UAPI_LINUX_ETHTOOL_NETLINK_H_ */

include/uapi/linux/ethtool_netlink_generated.h

···
 	ETHTOOL_MSG_KERNEL_MAX = (__ETHTOOL_MSG_KERNEL_CNT - 1)
 };
 
+#define ETHTOOL_MCGRP_MONITOR_NAME	"monitor"
+
 #endif /* _UAPI_LINUX_ETHTOOL_NETLINK_GENERATED_H */