Add reset control properties into MDIO nodes.  The 4 MDIO controllers in
AST2600 SOC share one reset control bit SCU50[3].
Signed-off-by: Dylan Hung <dylan_hung@aspeedtech.com>
Reviewed-by: Andrew Jeffery <andrew@aj.id.au>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>